GB1427340A - Data processing apparattus - Google Patents

Abstract

1427340 Data processing FERRANTI Ltd 20 Nov 1973 [21 Nov 1972] 53781/72 Heading G4A Data relating to topographic features represented by lines on a map are processed to identify intersections of the lines with a predefined line of sight and to determine the sense of the intersections. Cartesian co-ordinates of joints (points on a map line) are held in a store of a computer which is arranged to supply certain parameters relating the point of origin and angle # (to the nearest cardinal direction) of the sight line to the co-ordinates of a series of joints, e.g. along a particular contour line. The parameters for each joint are stored for processing in the logic unit shown, as follows:- Y in a counter C3, Ytan#-X (or equivalent expressions) in a counter C2 and a register R5, tan# in a register R4, a code representing the octant containing the line of sight in a register R3, and the joint number in a counter C1. The counter C1 includes further bits for addressing a store S2 to read out, into a register R1, words formed by pairs of bits which denote the direction, N, E, S or W of elements which together make up an approximation to the map line span between a pair of adjacent joints. A logic block CL2 combines each element bit pair and the octant code to derive outputs for adjusting the contents of C2, C3 and R5, e.g. by causing adder/subtractor AS to add the contents of R4 and R5, Ytan#-X may be changed to (Y + 1 )tan#-X in R5, and a carry produced during this operation is used to increment C2 which holds the high order bits of Ytan#-X. In this way a particular map line is followed from joint to joint. An intersection of the map line with the line of sight is detected by logic block CL3 when a change of sign occurs in C2, C3 and the value of Y then in C3 is transmitted to the computer via a register R6. The process is repeated for all elements and spans, R6 being updated if an intersection nearer to the point of origin is found. The sense of each intersection is determined by a logic block CL5. The process may be used to determine the inter-visibility of two points and for calculating field strength charts for radio transmitters.