Which parameter is used to correct the difference between grid length and geodetic length along a line?

• A. Scale factor
• B. Back azimuth
• C. Average elevation
• D. Forward direction

Answer: (A)

The important idea here is how map grid distances relate to true distances on the Earth’s surface. The scale factor tells you how much to adjust the grid length to obtain the geodetic length along a line. If you measure a line as L_grid on the grid, the true geodetic length along the ellipsoid is L_geodetic = L_grid × k, where k is the scale factor at that location (or along the line). A scale factor greater than 1 means the grid distance is too small to match the true distance, while a factor less than 1 means the grid distance is too large.

Back azimuth and forward direction are about direction, not length correction. Average elevation can affect 3D distances, but the direct correction between grid and geodetic lengths along a line on the grid relies on the scale factor.