Ground-to-grid combined scale factor explained (with geoid separation)
How grid scale factor, elevation factor, and geoid separation combine into the localization scale your CAD grid needs.
The short answer
The combined scale factor (CSF) reconciles ground distances with grid coordinates. It equals the grid scale factor (from your State Plane or UTM projection) multiplied by the elevation factor (from your height above the ellipsoid). Using orthometric elevation instead of ellipsoid height introduces up to ~5 ppm error in CONUS.
Grid scale factor
Every map projection distorts distances. On a State Plane Lambert Conformal Conic zone, the grid scale factor k varies with latitude and distance from the central meridian. At the standard parallels, k equals exactly 1. Between them, k is slightly less than 1.
FieldToCAD computes k at your survey centroid using the Krüger-series Transverse Mercator or Snyder LCC implementation with EPSG-verified zone parameters.
Elevation factor
The elevation factor reduces ground distances to the ellipsoid surface:
R / (R + h)
where R is the Gaussian mean radius of curvature at your latitude and h is ellipsoid height, not orthometric elevation.
Why geoid separation matters
Orthometric elevation H is height above the geoid (NAVD88). Ellipsoid height h = H + N, where N is geoid separation (GEOID18 in CONUS, roughly -20 to -35 m).
Ignoring N means your elevation factor uses the wrong height. At 1,600 m elevation in Colorado with N ≈ -30 m, the error is about 5 ppm, or 5 mm per km and 5 cm over a 10 km project.
FieldToCAD auto-fills geoid separation N from bundled NGS GEOID18 grids for CONUS, Puerto Rico, and the U.S. Virgin Islands. Manual N entry remains available when a project falls outside bundled coverage or when the surveyor needs to override the model value.
Putting it together
Your localization scale s should equal CSF = k × (R / (R + h)). FieldToCAD's auto-CSF tool computes this from your target zone, survey centroid, project elevation, and geoid separation.
The licensed surveyor should always review CSF before production export.