Wednesday, February 15, 2023

Why don't surveys and GIS match?

You may have wondered why surveyors in the 2020's don't deliver their dwg files on standard GIS  projection coordinates, whether UTM, US State Plane Coordinates, or something else. The short answer is that standard GIS projections usually don't give correct distances to the level of precision that real estate surveys need. I explain more below.


The sea level of the earth is roughly a squashed ball ellipsoid. But projections are like sheets of paper. A sheet of paper cannot be curled into a ball. But an approximate ball can be made with several strips of paper like paper mache. Each projection is one strip. Significantly, no projection fits the sea level earth perfectly. 


At any given point on any given projection, the projection is either outside or inside or exactly at the sea level earth. 

Illustration from

If it's outside, its distances are longer than earth distances. If it's inside, its distances are shorter than earth distances. So surveyors must scale projection coordinates so that their surveys give correct "ground" distances.

Scale factors

Every point on every projection has its own scale factor. For Maricopa County, Arizona, these factors are published all around the county, and in the Phoenix metropolitan area they are around 1.00016, meaning that the projection is inside the earth, and "ground" distances are 1.00016 times as long as projection distances. This scale factor makes a difference of about 1 foot per mile, which is not significant for most things, but is significant for real estate property lines.

Scaling maps

There is no set standard for scaling survey maps. Here are some possible strategies surveyors use:

Small projects

Scale map about a local point

For very small projects (1 acre or less), surveyors may leave the map on projection coordinates and scale the entire map about one corner or another. They may have an in-house standard for which corner they "hold" to projection coordinates. This introduces horizontal mapping errors of less than 1/2", which is acceptable for almost all mapping. An advantage of this approach is that it is generally transparent to end users. If it's clearly documented it can be a good approach.

General projects

Scale about the origin

For projects in general, they may scale their entire map about the projection system origin 0,0. Some surveyors feel that this puts the resulting map confusingly close to projection coordinates. It's best in this approach if the surveyor documents their scale factor or can provide it on request.

Scale and move

For projects in general, they may have a standard of moving their map an obvious amount in one direction. This can be confusing to the end user without clear documentation.

Scale at a new origin

For projects in general, they may have a standard of moving their map completely away from projection coordinates to near the origin 0,0 or some other place. This can be confusing to the end user without clear documentation.

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