Means for comparing values of pairs of said plurality of data point signals and providing a first logic signal when one of said pairs has equal values and a second logic signal when said one pair has nonequal values Įlevation averaging means for sequentially receiving said data point signals P1, P2, P3 and a further elevation data point signal P4 corresponding to an address (X i+1, Y i+1) from said memory means for providing a quotient signal corresponding to an average of said data point signals, said data point signals P1, P2, P3, P4 corresponding to predetermined integral X and Y coordinate positions representative of a plurality of displayable pixels on said map display surrounding a computed elevation data point signal P0 corresponding to said average value, said signal P0 comprising a fractional interval of said integral X and Y coordinate positions A contour line generator for overlaying an electronic map display and for defining topological features at predetermined intervals along a region of said map display, wherein said map display includes memory means for electronically storing digital elevation data representative of deviations from a predetermined plane of elevation at integral X and Y coordinate positions, means for providing digital data signals indicative of desired elevation contour lines, sequencer means responsive to a source of clock pulses for generating a plurality of timing control signals, and means for providing digital data signals indicative of a normal display of a region underlying said X and Y coordinate positions, said contour line generator comprising:Įdge detector means for sequentially receiving a plurality of digital elevation data point signals P1, P2, P3 from said memory means corresponding to addresses (X i, Y i), (X i +1, Y i), (X i, Y i +1) Exclusive property or right is claimed are defined as follows:ġ.
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