WO2010149854A1

WO2010149854A1 - Method and device for determination of distance

Info

Publication number: WO2010149854A1
Application number: PCT/FI2010/050531
Authority: WO
Inventors: Mikko Lindholm; Jouni Kaartinen
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 2009-06-26
Filing date: 2010-06-22
Publication date: 2010-12-29
Also published as: FI20095730A0; FI121440B

Abstract

A device for determining distance comprises a processing system (103) for performing triangulation and a direction sensor (104) for determining a position (α) with respect to a reference direction (105). Trigonometric function values for the triangulation are determined based on the position relative to the reference direction when the device is located at a first auxiliary point and directed towards the target, the position relative to the reference direction when the device is located at the first auxiliary point and directed towards a second auxiliary point, and the position relative to the reference direction when the device is located at the second auxiliary point and directed towards the target. It is not necessary to direct the device towards the first auxiliary point when the device is located at the second auxiliary point and, thus, it is not necessary for a user to remember, at the second auxiliary point, the exact location of the first auxiliary point.

Description

METHOD AND DEVICE FOR DETERMINATION OF DISTANCE

Field of the invention

The invention relates generally to a method and a device for determination of dis- tance. Furthermore, the invention relates to a computer program for determination of distance.

Background

Determination of distance can be based on, for example, the thangulation method, a laser time-of-flight measurement, a laser pointer pulse measurement, or a laser phase-shift measurement. A device for determination of distance on the basis of the triangulation is disclosed e.g. in publication US5914775, and a device for determination of distance on the basis of laser measurements is disclosed e.g. in publication US2009079954. An advantage of the triangulation when compared with the laser based methods is that there is no need to direct a laser beam to a target point to which the distance is wanted to be measured. In some cases, the laser beam may be disturbing or even dangerous for e.g. persons staying on the target point. On the other hand, a disadvantage of the triangulation compared with the lased based methods is that a measurement based on the triangulation cannot be done at a single point because a measurement triangle has to be constructed. In the triangulation, an estimate of a distance from a point A to a target point T can be determined with the aid of trigonometric calculations based on the following: (i) an estimate d_AB of a distance from the point A to a point B, (ii) an angle Φ_TAB between a first line from the point A to the target point T and a second line from the point A to the point B, and (iii) an angle Φ_TBA between the second line and a third line from the point B to the target point T.

In the traditional triangulation, the above-mentioned angle Φ_TAB is measured at the point A on the basis of the line from the point A to the target point T and the line from the point A to the point B. Correspondingly, the above-mentioned angle Φ_TBA is measured at the point B with the aid of the line from the point B to the target point T and the line from the point B to the point A. When the determination of distance is carried out by a single person using a portable device we can, without limiting the generality, assume that a measurer first measures the angle Φ_TAB at the point A and then moves to the point B to measure the angle Φ_TBA- An inconvenience related the above-described procedure is that the measurer has to either mark the point A, e.g. by using a landmark, or to remember the location of the point A after he has moved from the point A to the point B in order to be able to take an aim at the point B towards the point A. If the point A is situated on e.g. a flat field it may be necessary to mark the point A before moving from it to the point B.

Summary

In accordance with a first aspect of the invention there is provided a new device for determining a distance from a base point to a target. The device according to the invention comprises:

- a sight for directing the device towards a desired point,

a processing system for determining an estimate of the distance from the base point to the target on the basis of at least the following: (1 ) an estimate of a distance from a first auxiliary point to a second auxiliary point, (2) trigonometric function values for at least two angles of a measurement triangle defined by the target, the first auxiliary point, and the second auxiliary point, and (3) data indicating a location of the base point relative to the first and second auxiliary points, and

a direction sensor arranged to determine a position of the device with respect to a reference direction,

wherein the processing system is arranged to determine the trigonometric function values on the basis of (i) the position of the device with respect to a reference direction when the device is located at the first auxiliary point and directed towards the target, (ii) the position of the device with respect to the reference direction when the device is located at the first auxiliary point and directed towards the sec- ond auxiliary point, and (iii) the position of the device with respect to the reference direction when the device is located at the second auxiliary point and directed towards the target.

As the above-mentioned trigonometric function values are determined on the basis of the position of the device with respect to the reference direction in the above- described situations (i-iii), it is not necessary to direct the device towards the first auxiliary point when the device is located at the second auxiliary point. Hence, it is not necessary for a user to mark the first auxiliary point or to remember, after mov- ing from the first auxiliary point to the second auxiliary point, the exact location of the first auxiliary point.

The direction sensor can comprise, for example, an electrical magnetometer or a compass including a rotatably pivoted magnet. In this case, the reference direction is fixed to the direction of the magnetic field produced by the Earth. The sight of the device can be, for example, as simple as a sufficiently straight edge of the housing of the device or an advanced optical sight.

In accordance with a second aspect of the invention there is provided a new method for determining a distance from a base point to a target. The method ac- cording to the invention comprises:

at a first auxiliary point, directing a device towards the target,

at the first auxiliary point, directing the device towards a second auxiliary point,

moving the device from the first auxiliary point to the second auxiliary point,

at the second auxiliary point, directing the device towards the target, and

determining trigonometric function values for at least two angles of a measurement triangle defined by the target, the first auxiliary point, and the second auxiliary point on the basis of (i) the position of the device with respect to a reference direction when the device is located at the first auxiliary point and directed towards the target, (ii) the position of the device with respect to the reference direction when the device is located at the first auxiliary point and directed towards the second auxiliary point, and (iii) the posi- tion of the device with respect to the reference direction when the device is located at the second auxiliary point and directed towards the target, and

determining an estimate of the distance from the base point to the target on the basis of at least the following: (i) an estimate of a distance from the first auxiliary point to the second auxiliary point, (ii) the trigonometric function values, and (iii) data indicating a location of the base point relative to the first and second auxiliary points. In accordance with a third aspect of the invention there is provided a new computer program for determining a distance from a base point to a target when run in a programmable processor of a device comprising a sight and a direction sensor. The computer program according to the invention comprises computer executable instructions for controlling the programmable processor to:

determine trigonometric function values for at least two angles of a measurement triangle defined by the target, a first auxiliary point, and a second auxiliary point on the basis of (i) the position of the device with respect to a reference direction when the device is located at the first auxiliary point and directed towards the target, (ii) the position of the device with respect to the reference direction when the device is located at the first auxiliary point and directed towards the second auxiliary point, and (iii) the position of the device with respect to the reference direction when the device is located at the second auxiliary point and directed towards the target, and

- determine an estimate of the distance from the base point to the target on the basis of at least the following: (i) an estimate of a distance from the first auxiliary point to the second auxiliary point, (ii) the trigonometric function values, and (iii) data indicating a location of the base point relative to the first and second auxiliary points.

A computer program product according to the invention comprises a computer readable medium, e.g. a compact disc (CD) or a random access memory (RAM), encoded with a computer program according to the invention.

A number of exemplifying embodiments of the invention are described in accompanied dependent claims.

Various exemplifying embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying embodiments when read in connection with the accompanying drawings.

The verb "to comprise" is used in this document as an open limitation that neither excludes nor requires the existence of unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Brief description of the figures

The exemplifying embodiments of the invention and their advantages are explained in greater detail below with reference to the accompanying drawings, in which:

figure 1 shows a schematic illustration of a device according to an embodiment of the invention for determining a distance,

figure 2 illustrates a principle utilized in a device according to an embodiment of the invention for determining a distance,

figures 3a and 3b illustrate a principle utilized in a device according to an embodi- ment of the invention for determining a distance, and

figure 4 shows a flow chart of a method according to an embodiment of the invention for determining a distance.

Description of the exemplifying embodiments

Figure 1 shows a schematic illustration of a device according to an embodiment of the invention for determining distance. The device comprises a sight for directing the device towards a desired point. The sight of the device can be, for example, as simple as a sufficiently straight edge of the housing of the device. Figure 1 shows an exemplifying case in which the sight comprises a front sight 102 and a back sight 101 fastened to a surface of the device. It is also possible to provide the de- vice with an optical sight. It is also possible that the sight comprises a digital camera and a display screen arranged to show a view captured by the digital camera. The display screen may comprise a graticule for facilitating the aiming. The device comprises a direction sensor 104 arranged to determine a position of the device with respect to a reference direction 105. In the exemplifying situation shown in figure 1 , a user 109 has directed the device towards an object 107. In the exemplifying situation shown in figure 1 , the position of the device with respect to the reference direction 105 is expressed with the aid of an angle α between the reference direction and a sight rail 110, wherein the sight rail is a line along the sight of the device. The direction sensor 104 may comprise, for example, a magnetometer that is arranged to determine the position of the device with respect to the reference direction 105 and to deliver the measured position information to a processing system 103. In this case, the reference direction 105 is fixed to the direction of the magnetic field produced by the Earth. The direction sensor 104 can as well comprise a compass including a rotatably pivoted magnet, and a circuitry for delivering position information measured by the compass to the processing system. Also in this case, the reference direction is fixed to the direction of the magnetic field produced by the Earth. The processing system 103 can contain one or more processor units.

The operation of the processing system 103 in a device according to an exemplifying embodiment of the invention is illustrated below with reference to figure 2 that depicts a measurement triangle defined by a target T, a first auxiliary point A, and a second auxiliary point B. The processing system is arranged to determine trigo- nometric function values for angles Φ_A and Φ_T of the measurement triangle on the basis of the following:

the position α1 of the device with respect to the reference direction 205 when the device is located at the first auxiliary point A and directed towards the target T,

- the position α2 of the device with respect to the reference direction 205 when the device is located at the first auxiliary point A and directed towards the second auxiliary point B, and

the position α3 of the device with respect to the reference direction 205 when the device is located at the second auxiliary point B and directed to- wards the target T.

In this exemplifying case we assume for the sake of illustrative purposes that all the angles shown in figure 2 are on the same plane. A more advanced embodiment of the invention in conjunction of which this assumption is not necessary will be presented later in this document. It can be seen from figure 2 that φ_A = α1 - α2 and Φ_T = α3 - α1 , because the angles α1 , α2, and α3 are assumed to be on the same plane. The processing system 103 is arranged to determine an estimate dβ_aτ of a distance from the base point Ba to the target T on the basis of:

an estimate d_Aβ of a distance from the point A to the point B,

the trigonometric function values sin(φ_A) and sin(φ_τ) determined for the angles φ_A and φ_τ, and

data indicating the location of the base point Ba relative to the points A and B. The data that indicates the location of the base point Ba relative to the points A and B may indicate, for example, a distance dββa from the point B to the base point Ba and an angle ε shown in figure 2. As another example, the above-mentioned data may indicate a distance dABa between the point A and the base point Ba and the distance dββa from the point B to the base point Ba. It should be noted that the above-presented are only examples and there are numerous ways to indicate the location of the base point Ba relative to the points A and B. It should be also noted that the base point Ba may in some cases be one or the other of the points A and B.

The estimate dβ_aτ of the distance from the base point Ba to the target T can be determined, for example, according to the following equations (1 ) and (2): sin(φ_A ) d^BT - ^dAB sin(φ_τ ) ' ^{(1 )}

d_Baτ = V^d _Bτ² + d_BBa ²- 2d_BTd_BBacos(φ_B + ε) , (2)

where dβT is an estimate of the distance from the point B to the target T, dββa is the known distance from the point B to the base point Ba, ε is the known angle shown in figure 2, and φβ = π - φ-r - Φ_A- Equation (1 ) contains the trigonometric function values sin(φ_A) and sin(φ_τ) for the angles ΦA and φj. It should be, however, noted that it is possible to use also other trigonometric functions than the sine function since the trigonometric functions are interrelated with each other in known man- ners, e.g. sin(φ_A) = V(1 - cos²(φ_A)).

The operation of a device according to another exemplifying embodiment of the invention is illustrated below with reference to figures 3a and 3b. In this case, there is no need to make any assumptions such that all the angles should be in the same plane. Hence, the device whose operation is illustrated below is suitable for three dimensional operating environments. The direction sensor of the device is arranged to indicate an angle between a sight rail and the direction of the gravity and an angle between a horizontal projection of the sight rail and the reference direction, where the sight rail is the line along the sight of the device. In figure 3a, the angle between the sight rail 310 and the direction of the gravity g is denoted with θ, and the angle between the horizontal projection of the sight rail and the reference direction 305 is denoted with φ. In figure 3a, S represents a unit vector having the direction of the sight rail 310. Figure 3a shows also a vector base 320 that contains orthogonal unit vectors i, j, and k. As shown in figure 3b, the unit vector i is oriented along the reference direction 305 and the unit vector k is oriented to be opposite to the gravity g. The unit vector j is defined as the vector product k x i, i.e. the cross product. The unit vector S can be expressed as:

S = isin(θ)cos(φ) + jsin(θ)sin(φ) + kcos(θ). (3)

Figure 3b shows an exemplifying situation in which a distance from a base point Ba to a target T is to be determined. The target T, a first auxiliary point A, and a second auxiliary point B constitute a measurement triangle. It should be noted that nothing prevents from using the base point Ba as one of the auxiliary points. At the first auxiliary point A, the device is directed towards the target T. The direction sensor of the device measures the angles θ and φ shown in figure 3a and the processing system of the device forms a unit vector Si according to equation (3). A unit vector S₂ is formed in the corresponding manner when the device is at the point A and directed towards the second auxiliary point B, and a unit vector S3 is formed in the corresponding manner when the device is at the point B and directed towards the target T. In this exemplifying situation it is assumed that a unit vector S_B and a distance dββa from the point B to the base point Ba are known, i.e. data indicating the location of the base point Ba relative to the points A and B contains information about the unit vector S_B and about the distance dββa- The sine of an¬

sin(φ_A) = abs(Si x S₂), sin(φ_B) = abs(S₃ ^x - S₂), (4)

where '*' means the vector product, i.e. the cross product, and 'abs' means the absolute value, i.e. the length of a vector. The values of sin(φ_A) and sin(φ_B) are greater than zero because Φ_A and φβ are greater than zero and less than π. The cosine of angles φ_A and φ_B are:

COS(ΦA) = Si ^■ S₂, cos(φβ) = S₃ - S₂, (5)

where '^■' means the scalar product, i.e. the dot product. The sine of the angle Φ_T is:

sin(φτ) = sin(π - (φ_A + ΦB)) = sin(φ_A + ΦB) = sin(φ_A) cos(φ_B) + cos(φ_A)sin(φ_B).

(6)

The estimate dβT of the distance from the point B to the target T can be deter- mined, for example, with the aid of the estimate d_Aβ of the distance from the point A to the point B, the trigonometric function values sin(φ_A) and sin(φ_τ), and the equation (1 ) that was presented earlier in this document. The estimate dβ_aτ of the distance from the base point Ba to the target T can be determined, for example, according to the following equations (7) and (8):

dβ_aT = V^d _BT ² + d_BBa ²- ²d_BA_BaCOS(⁽t>_Ba ) , (8)

where dβT is the estimate of the distance from the point B to the target T, dββa is the known distance from the point B to the base point Ba, and S_B is the known unit vector pointing from the point B towards the base point Ba.

The distance values in the equations (1 ), (2), and (8) can be expressed in any unit, e.g. in meters, in intervals of successive telephone poles, in steps, etc. The dis- tance estimate dβ_aτ given by equation (2) or (8) is expressed in the same unit, e.g. in meters, in intervals of successive telephone poles, in steps, ..., as the estimate dAB of the distance from the point A to the point B and the distance dββa from the point B to the base point Ba. The estimate dAB of the distance from the point A to the point B can be measured when moving from the point A to the point B using some suitable apparatus such as e.g. a milometer of a car or a bicycle, or the estimate dAB can formed by ocular inspection.

In a device according to an embodiment of the invention, the direction sensor comprises a magnetometer arranged to indicate the angle φ shown in figure 3a, and an acceleration sensor arranged to indicate the angle θ shown in figure 3a. The processing system of the device is preferably arranged to correct the value of the angle φ on the basis of the angle θ.

In a device according to an embodiment of the invention, the direction sensor comprises a magnetometer rotatably supported to the device with two mutually intersecting shafts and weighted in order to keep the magnetometer in a constant orientation with respect to the direction of the gravity. The magnetometer is arranged to indicate the angle φ shown in figure 3a, and the orientation of the magnetometer with respect to the sight rail is arranged to indicate the angle θ shown in figure 3a.

In a device according to an embodiment of the invention, the direction sensor comprises a compass rotatably supported to the device with two mutually intersecting shafts and weighted in order to keep the compass in a constant orientation with respect to the direction of the gravity. The compass is arranged to indicate the angle φ shown in figure 3a, and the orientation of the compass with respect to the sight rail is arranged to indicate the angle θ shown in figure 3a.

A device according to an embodiment of the invention comprises a reception module of a satellite positioning system. In figure 1 , a block 106 represents the reception module that is arranged to receive positioning signals from satellites 108 of the satellite positioning system. The processing system 103 is arranged to calculate the estimate dAB of the distance from the first auxiliary point A shown in figures 2 and 3b to the second auxiliary point B shown in figures 2 and 3b on the basis of an output signal of the reception module in a situation in which the device has been moved from the point A to the point B. The satellite positioning system can be the Global Positioning System (GPS) maintained by the United States of America, the European Galileo positioning system, or the Russian GLONASS positioning system.

A device according to an embodiment of the invention comprises an acceleration sensor and the processing system is arranged to calculate the estimate dAB of the distance from the first auxiliary point A to the second auxiliary point B on the basis of an output signal of the acceleration sensor in a situation in which the device is moved from the first auxiliary point to the second auxiliary point. As acceleration is the second time derivative of displacement, the distance estimate dAB can be de- termined by integrating the measured acceleration two times with respect to time.

A device according to an embodiment of the invention comprises an acceleration sensor arranged detect steps taken by a user and the processing system is arranged to count the number of steps as a response to a situation in which the user moves from the first auxiliary point A to the second auxiliary point B. In this case, the estimate dAB of the distance from the point A to the point B can be expressed in steps.

A device according to an embodiment of the invention comprises at least one of the following: a digital camera, a mobile communication device, a telescope, a binocular.

Figure 4 shows a flow chart of a method according to an embodiment of the invention for determining a distance from a base point Ba to a target T. The method comprises: at a first auxiliary point A, directing 401 a device towards the target T and determining 402 the position of the device with respect to a reference direction R,

at the first auxiliary point A, directing 403 the device towards a second auxiliary point B and determining 404 the position of the device with respect to a reference direction R,

moving 405 the device from the first auxiliary point A to the second auxiliary point B,

at the second auxiliary point B, directing 406 the device towards the target T and determining 407 the position of the device with respect to the reference direction R,

determining 408 trigonometric function values for at least two angles of a measurement triangle defined by the target T, the first auxiliary point A, and the second auxiliary point B on the basis of (i) the position of the device with respect to a reference direction R when the device is located at the first auxiliary point A and directed towards the target T, (ii) the position of the device with respect to the reference direction R when the device is located at the first auxiliary point A and directed towards the second auxiliary point B, and (iii) the position of the device with respect to the reference direction R when the device is located at the second auxiliary point B and directed towards the target T, and

determining 409 an estimate of the distance from the base point Ba to the target T on the basis of at least the following: (i) an estimate of a distance from the first auxiliary point A to the second auxiliary point B, (ii) the trigonometric function values, and (iii) data indicating a location of the base point relative to the first and second auxiliary points A and B.

In a method according to an embodiment of the invention, the position of the device with respect to the reference direction R is measured with a magnetometer and the reference direction R is fixed to the direction of the magnetic field pro- duced by the Earth.

In a method according to an embodiment of the invention, the position of the device with respect to the reference direction R is measured with a compass includ- ing a rotatably pivoted magnet and the reference direction R is fixed to the direction of the magnetic field produced by the Earth.

In a method according to an embodiment of the invention, the device comprises a reception module of a satellite positioning system and the estimate of the distance from the first auxiliary point A to the second auxiliary point B is calculated on the basis of an output signal of the reception module when the device has been moved from the first auxiliary point A to the second auxiliary point B.

In a method according to an embodiment of the invention, the satellite positioning system is one of the following: the Global Positioning System maintained by the United States of America, the European Galileo positioning system, the Russian GLONASS positioning system.

In a method according to an embodiment of the invention, a sight comprising a front sight and a back sight fastened to a surface of the device is used for directing the device towards the target T and for directing the device towards the second auxiliary point B.

In a method according to an embodiment of the invention, a sight comprising an optical sight is used for directing the device towards the target T and for directing the device towards the second auxiliary point B.

In a method according to an embodiment of the invention, a digital camera and a display screen arranged to show a view captured by the digital camera are used for directing the device towards the target T and for directing the device towards the second auxiliary point B.

In a method according to an embodiment of the invention, the position of the device with respect to the reference direction is determined by measuring an angle between a sight rail and the direction of the gravity and an angle between a horizontal projection of the sight rail and the reference direction, the sight rail being a line along a sight of the device.

In a method according to an embodiment of the invention, the angle between the horizontal projection of the sight rail and the reference direction is measured with a magnetometer, the angle between the sight rail and the direction of the gravity is measured with an acceleration sensor, and the value of the angle measured with the magnetometer is corrected on the basis of the angle measured with the acceleration sensor. In a method according to an embodiment of the invention, the angle between the horizontal projection of the sight rail and the reference direction is measured with a magnetometer rotatably supported to the device with two mutually intersecting shafts and weighted in order to keep the magnetometer in a constant orientation with respect to the direction of the gravity, the orientation of the magnetometer with respect to the sight rail being arranged to indicate the angle between the sight rail and the direction of the gravity.

In a method according to an embodiment of the invention, the angle between the horizontal projection of the sight rail and the reference direction is measured with a compass rotatably supported to the device with two mutually intersecting shafts and weighted in order to keep the compass in a constant orientation with respect to the direction of the gravity, the orientation of the compass with respect to the sight rail being arranged to indicate the angle between the sight rail and the direction of the gravity.

In a method according to an embodiment of the invention, the device comprises an acceleration sensor. The estimate of the distance from the first auxiliary point A to the second auxiliary point B is calculated on the basis of an output signal of the acceleration sensor when the device is moved from the first auxiliary point A to the second auxiliary point B.

In a method according to an embodiment of the invention, the device comprises at least one of the following: a digital camera, a mobile communication device, a telescope, a binocular.

A computer program according to an embodiment of the invention comprises a program code for determining a distance from a base point to a target when run in a programmable processor of a device comprising a sight and a direction sensor. The program code comprises computer executable instructions for controlling the programmable processor to:

determine trigonometric function values for at least two angles of a measurement triangle defined by the target, a first auxiliary point, and a second auxiliary point on the basis of (i) the position of the device with respect to a reference direction when the device is located at the first auxiliary point and directed towards the target, (ii) the position of the device with respect to the reference direction when the device is located at the first auxiliary point and directed towards the second auxiliary point, and (iii) the posi- tion of the device with respect to the reference direction when the device is located at the second auxiliary point and directed towards the target, and

determine an estimate of the distance from the base point to the target on the basis of at least the following: (i) an estimate of a distance from the first auxiliary point to the second auxiliary point, (ii) the trigonometric function values, and (iii) data indicating a location of the base point relative to the first and second auxiliary points.

The computer executable instructions can be e.g. subroutines and/or functions.

A computer readable medium, e.g. a CD-ROM (Compact Disc Read Only Mem- ory) or a RAM-device (Random Access Memory), according to an embodiment of the invention is encoded with a computer program according to an embodiment of the invention.

A signal according to an embodiment of the invention is encoded with a computer program according to an embodiment of the invention.

The specific examples provided in the description given above should not be construed as limiting. Therefore, the invention is not limited merely to the embodiments described above, many variants being possible.

Claims

Claims:

1. A device for determining a distance from a base point to a target, the device comprising:

a sight (101 , 102) for directing the device towards a desired point,

- a processing system (103) for determining an estimate of the distance from the base point to the target on the basis of at least the following: (i) an estimate of a distance from a first auxiliary point to a second auxiliary point, (ii) trigonometric function values for at least two angles of a measurement triangle defined by the target, the first auxiliary point, and the second auxil- iary point, and (iii) data indicating a location of the base point relative to the first and second auxiliary points,

characterized in that the device further comprises a direction sensor (104) arranged to determine a position of the device with respect to a reference direction (105), and the processing system is arranged to determine the trigonometric func- tion values on the basis of (i) the position of the device with respect to a reference direction when the device is located at the first auxiliary point and directed towards the target, (ii) the position of the device with respect to the reference direction when the device is located at the first auxiliary point and directed towards the second auxiliary point, and (iii) the position of the device with respect to the reference direction when the device is located at the second auxiliary point and directed towards the target.

2. A device according to claim 1 , wherein the direction sensor comprises a magnetometer and the reference direction is fixed to the direction of the magnetic field produced by the Earth.

3. A device according to claim 1 , wherein the direction sensor comprises a compass including a rotatably pivoted magnet, and a circuitry for delivering position information measured by the compass to the processing system, the reference direction being fixed to the direction of the magnetic field produced by the Earth.

4. A device according to claim 1 , comprising a reception module (106) of a satellite positioning system, and the processing system is arranged to calculate the estimate of the distance from the first auxiliary point to the second auxiliary point on the basis of an output signal of the reception module in a situation in which the device has been moved from the first auxiliary point to the second auxiliary point.

5. A device according to claim 4, wherein the satellite positioning system is one of the following: the Global Positioning System maintained by the United States of America, the European Galileo positioning system, the Russian GLONASS positioning system.

6. A device according to claim 1 , wherein the sight comprises a front sight (102) and a back sight (101 ) fastened to a surface of the device.

7. A device according to claim 1 , wherein the sight comprises an optical sight.

8. A device according to claim 1 , wherein the sight comprises a digital camera and a display screen arranged to show a view captured by the digital camera.

9. A device according to claim 1 , wherein the direction sensor is arranged to indicate an angle between a sight rail and the direction of the gravity and an angle between a horizontal projection of the sight rail and the reference direction, the sight rail being a line along the sight of the device.

10. A device according to claim 9, wherein the direction sensor comprises a mag- netometer arranged to indicate the angle between the horizontal projection of the sight rail and the reference direction and an acceleration sensor arranged to indicate the angle between the sight rail and the direction of the gravity, and the processing system is arranged to correct the value of the angle indicated with the magnetometer on the basis of the angle indicated with the acceleration sensor.

11. A device according to claim 9, wherein the direction sensor comprises a magnetometer rotatably supported to the device with two mutually intersecting shafts and weighted in order to keep the magnetometer in a constant orientation with respect to the direction of the gravity, the magnetometer being arranged to indicate the angle between the horizontal projection of the sight rail and the reference di- rection, and the orientation of the magnetometer with respect to the sight rail being arranged to indicate the angle between the sight rail and the direction of the gravity.

12. A device according to any of claims 1 -11 , wherein the device comprises at least one of the following: a digital camera, a mobile communication device, a telescope, a binocular.

13. A device according to claim 1 , wherein the device comprises an acceleration sensor and the processing system is arranged to calculate the estimate of the dis- tance from the first auxiliary point to the second auxiliary point on the basis of an output signal of the acceleration sensor in a situation in which the device is moved from the first auxiliary point to the second auxiliary point.

14. A method for determining a distance from a base point to a target, the method comprising:

at a first auxiliary point, directing (401 ) a device towards the target,

at the first auxiliary point, directing (403) the device towards a second auxiliary point,

moving (405) the device from the first auxiliary point to the second auxiliary point,

at the second auxiliary point, directing (406) the device towards the target, and

determining (409) an estimate of the distance from the base point to the target on the basis of at least the following: (i) an estimate of a distance from the first auxiliary point to the second auxiliary point, (ii) trigonometric function values for at least two angles of a measurement triangle defined by the target, the first auxiliary point, and the second auxiliary point, and (iii) data indicating a location of the base point relative to the first and second auxiliary points,

characterized in that the trigonometric function values are determined (402, 404, 407, 408) on the basis of (i) the position of the device with respect to a reference direction when the device is located at the first auxiliary point and directed towards the target, (ii) the position of the device with respect to the reference direction when the device is located at the first auxiliary point and directed towards the sec- ond auxiliary point, and (iii) the position of the device with respect to the reference direction when the device is located at the second auxiliary point and directed towards the target.

15. A method according to claim 14, wherein the position of the device with respect to the reference direction is measured with a magnetometer and the refer- ence direction is fixed to the direction of the magnetic field produced by the Earth.

16. A method according to claim 14, wherein the position of the device with respect to the reference direction is measured with a compass including a rotatably pivoted magnet and the reference direction is fixed to the direction of the magnetic field produced by the Earth.

17. A method according to claim 14, wherein the device comprises a reception module of a satellite positioning system and the estimate of the distance from the first auxiliary point to the second auxiliary point is calculated on the basis of an output signal of the reception module when the device has been moved from the first auxiliary point to the second auxiliary point.

18. A method according to claim 17, wherein the satellite positioning system is one of the following: the Global Positioning System maintained by the United States of America, the European Galileo positioning system, the Russian GLONASS positioning system.

19. A method according to claim 14, wherein a sight comprising a front sight and a back sight fastened to a surface of the device is used for directing the device towards the target and for directing the device towards the second auxiliary point.

20. A method according to claim 14, wherein a sight comprising an optical sight is used for directing the device towards the target and for directing the device towards the second auxiliary point.

21. A method according to claim 14, wherein a digital camera and a display screen arranged to show a view captured by the digital camera are used for directing the device towards the target and for directing the device towards the second auxiliary point.

22. A method according to claim 14, wherein the position of the device with respect to the reference direction is determined by measuring an angle between a sight rail and the direction of the gravity and an angle between a horizontal projec- tion of the sight rail and the reference direction, the sight rail being a line along a sight of the device.

23. A method according to claim 22, wherein the angle between the horizontal projection of the sight rail and the reference direction is measured with a magnetometer, the angle between the sight rail and the direction of the gravity is measured with an acceleration sensor, and the value of the angle measured with the magnetometer is corrected on the basis of the angle measured with the acceleration sensor.

24. A method according to claim 22, wherein the angle between the horizontal projection of the sight rail and the reference direction is measured with a magnetometer rotatably supported to the device with two mutually intersecting shafts and weighted in order to keep the magnetometer in a constant orientation with respect to the direction of the gravity, the orientation of the magnetometer with respect to the sight rail being arranged to indicate the angle between the sight rail and the direction of the gravity.

25. A method according to any of the claims 14-24, wherein the device comprises at least one of the following: a digital camera, a mobile communication device, a telescope, a binocular.

26. A method according to claim 14, wherein the device comprises an acceleration sensor and the estimate of the distance from the first auxiliary point to the second auxiliary point is calculated on the basis of an output signal of the acceleration sensor when the device is moved from the first auxiliary point to the second auxil- iary point.

27. A computer program for determining a distance from a base point to a target when run in a programmable processor of a device comprising a sight and a direction sensor, the computer program comprising computer executable instructions for controlling the programmable processor to determine an estimate of the dis- tance from the base point to the target on the basis of at least the following: (i) an estimate of a distance from a first auxiliary point to a second auxiliary point, (ii) trigonometric function values for at least two angles of a measurement triangle defined by the target, the first auxiliary point, and the second auxiliary point, and (iii) data indicating a location of the base point relative to the first and second auxil- iary points, characterized in that the computer program further comprises computer executable instructions for controlling the programmable processor to determine the trigonometric function values on the basis of (i) the position of the device with respect to a reference direction when the device is located at the first auxiliary point and directed towards the target, (ii) the position of the device with respect to the reference direction when the device is located at the first auxiliary point and directed towards the second auxiliary point, and (iii) the position of the device with respect to the reference direction when the device is located at the second auxiliary point and directed towards the target.

28. A computer readable medium, characterized in that the computer readable medium is encoded with a computer program according to claim 27.