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Nikita Desjardins

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Aug 3, 2024, 5:41:27 PM8/3/24

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Which is better to use to report area/line lengths? I'm working in western North Carolina in ArcGIS Pro using NAD_1983_StatePlane_North_Carolina_FIPS_3200_Feet. I notice using the "Measure Features" tool that the area differences (between Geodesic and Planar) are nearly non-existent over small areas, but 4 acres different over a 20,000 acre area.

Does the "Measure Features" tool default to the coordinate system of the map? I had a map that was in WGS84, and was getting some seemingly very wrong answers, presumably because it was unable to calculate area in the Geographic Coordinate system?

1. If you have a projected coordinate system, planar is typically chosen, especially with lines are you are most likely looking for point a to point b regardless of the shape of the Earth. If you have just a geographic coordinate system, you can only use geodesic.

That was the case in Desktop. But in Pro, even with the Maps's display coordinate system set to a projected CS, the only area option is geodesic. I am an instructor and want to fully understand this change.

Don't forget, an ellipsoid is only an approximation of Earth's shape, and area and length calculations are best estimates. Simply consider a field with a 20 degree slope. It doesn't matter if you measure area using geodesic methods from long/lat coordinates of the corners, or projected coordinates in planar measures... both will be 'wrong' because the 3D area is going to be greater than the 2D area whether it is a geodesic or planar measure.

I get what you are saying, both types of measurement are abstractions from reality. But what I'm trying to understand is whether 'planar' measurements are necessary now that ArcPro is measuring area and distance geodesically. ArcMap Desktop wasn't able to make geodesic calculations (area - disabled if only geographic coordinate systems were used in the data frame). But now, even with a projected CS in the map's display, the only option for area measurement is geodesic (which it should be because it will always be more accurate than a flattened model of the ellipse).

Drop the notion that geodesic is 'more accurate'. That completely depends on the size of the study area and the projection being used. If you are working on a province/state wide study, then the 'extents' are critical. Also, what do you mean by 'accurate' perhaps %age wise? Most projections that are used wisely, will have a specified value?.

To make things worse, they will try to measure things on Google Earth or AGOL using a 'webbie' projections like Web Mercator, which is the last thing one should use (but everyone uses it don't they !?!? )

Precision and accuracy... slippery concepts. How well do we really know the shape of earth? 1927 to 1983 big leap. New ones on the horizon? I suspect yes. It may improve locational precision, but a real reflection of land based distances... no, since distance will be a traversal along an ellipsoid.

I think there might be some kind of glitch. If you have time, check out the attached images. I don't have the option to create planar calculations in ArcGIS Pro. You can see that the map's display CS is set to a projected one yet I only have the option to calculate area and perimeter geodesically.

I've taught coordinate systems in ArcMap Desktop for 10 years. I'm learning ArcGIS Pro and switching my material over. From my experience with it (and the student who is working with me) it seemed as though ArcPro had done away with planar measurements, yet I found no mention of this on forums, documentation, etc. Seemed like a pretty big change. But now I'm wondering if there is something glitching with my system as you are telling me that geodesic should be an 'option' not the only choice...

otherwise things like a portion of a UTM zone ( ie -78 to -72 long. by 42 to 48 labt.) gets distorted if you project it without densification. The longitudes will appear fine, but the north and south latitudes bands will not have the appropriate curvature unless you densify.

This does not seem to be the case for all PCS. For example, a Mercator projected data layer does not offer a planar area option in the Calculate Geometry Attributes tool. Yet, many other non-area preserving cylindrical coordinate systems like Miller do offer the planar area option. This seems to be true as of version 2.4.2.