Ground and local coordinates

State plane and UTM coordinates are grid coordinates: a map projection stretches or shrinks distances a little (tens of parts per million), and the higher the site, the more a measured ground distance differs from the grid one. Many surveys therefore hand over ground coordinates, scaled so plan distances equal distances measured on the ground, or an assumed site grid (5000, 5000 at a control point). CivilCraft keeps your numbers as they are and records how they relate to the grid, so latitude and longitude, GIS files and map layers still land in the right place.

Open Coordinates in the GIS group, pick (or keep) the system, then choose under Grid, ground or local coordinates:

Whatever you choose is saved with the project and is one undo step; nothing moves.

Ground coordinates

1. Choose Ground.
2. Type the Combined scale factor from the survey (for example 0.99987), or compute it:
   1. Under Compute the factor at a point, the point starts at the middle of the project. Choose Pick a point on the plan and click the spot (a click near a survey point takes that point), or type E and N.
   2. The elevation comes from the surface there; type another one if you need to.
   3. Choose Compute. CivilCraft shows:
   4. Grid scale factor k: how much the map projection stretches distances there.
   5. Elevation factor: how much shorter distances are at the ellipsoid than at that height.
   6. Combined factor = k × elevation factor: grid distance = ground distance × this.
   7. Convergence: the angle from grid north to true north there (geodetic azimuth = grid azimuth + convergence).
   8. Choose the Use button under the numbers to put the combined factor in the box.
3. Set the Base point, where ground and grid coordinates are the same. It starts at 0, 0: most jurisdictions that publish ground coordinates scale about the origin, so ground coordinates stay recognizably close to grid ones. Some surveys scale about the project's control point instead: use what the survey says.
4. Choose Apply.

Local (assumed) coordinates

1. Choose Local.
2. For Control point 1, type the survey point's number (or choose Pick and click it on the plan). Its local coordinates show beside it.
3. Type its grid E and N, or its latitude and longitude in the third box (for example 30.2672, -97.7431).
4. Do the same for Control point 2, as far from the first as you can.
5. CivilCraft shows the Scale and the Rotation found, and reminds you: Two points always fit exactly — check a third.
6. Give a Check point the same way. CivilCraft shows how far the fit puts it from the grid coordinates you typed (its misclosure, in project units and parts per million). A large misclosure means a wrong point, a typing error, or coordinates in another unit.
7. Choose Apply.

A scale far from 1 (beyond 0.5 to 2) is refused: check the points and their units.

What was checked

• The scale factor and convergence match PROJ (pyproj 3.7.1 with PROJ 9.5.1), the coordinate library most GIS programs use, to 1 part in a billion on over 300 systems with conformal projections (state plane, UTM, transverse Mercator, Lambert conformal conic), and in the browser at a point picked on the plan.
• Ground and local steps convert back and forth to 1 part in a billion; two control points give back the scale and rotation they were made with; a check point 0.05 ft off shows a misclosure of 0.05 ft.