Does the projection of my Map affect area and distance calculations by Geoprocessing tools?

[LucasBeck]

Hi everyone,

Does the projection of my map affect area and distance calculations made by geoprocessing tools or do these tools use the projected coordinate system of the layers involved in the calculation (e.g., would a changing the map projection change the results from using the summarizing within tool if the layer projections remained the same?). Thank you all.

Best,

Lucas 

[DanPatterson]

Yes the projection does, use geodesic calculations where possible and/or stick to equal area, equal distance or well know conformal projections like UTM, state Plane etc.

Don't ever use a Web Mercator projection for area/distance calculations or they will be wrong.  Google the topic to see what I mean

[LucasBeck]

Hi Dan,

Thanks, so the tools will use the map projection instead of the projected coordinate system associated with the layer? Thanks again

[DanPatterson]

Note

Calculate Geometry Attributes (Data Management)—ArcGIS Pro | Documentation

Geoprocessing considerations for length and area units—ArcGIS Pro | Documentation

you can specify the coordinate system to use in the calculation and the considerations to make while doing so.

[ChristopherCounsell]

Yes. But you can also run the tools without a map. And some tools behave differently. Not all geoprocessing tools will honour the environment settings. In these scenarios it may default to the input data or apply a default. It is best to read the general ArcGIS Pro environment settings and the tool page to understand the behaviour.

This page covers the topic directly:

https://pro.arcgis.com/en/pro-app/latest/tool-reference/appendices/spatial-reference-and-geoprocessi...

Other useful pages:

https://pro.arcgis.com/en/pro-app/latest/tool-reference/environment-settings/an-overview-of-geoproce...

https://pro.arcgis.com/en/pro-app/latest/tool-reference/environment-settings/cartographic-coordinate...

e.g. running the Append tool, if your input data coordinate system is different to the target dataset, it will be projected. But with what transformation method? If a different method is desired you may need to project the data first (where a transformation method can be specified) and then append.

https://pro.arcgis.com/en/pro-app/latest/tool-reference/data-management/append.htm

Other tools you can specify as parameters e.g. transformations in feature class to feature class

https://pro.arcgis.com/en/pro-app/latest/tool-reference/conversion/feature-class-to-feature-class.ht...

The summarize within tool lets you specify the output coordinate system and transformations:

https://pro.arcgis.com/en/pro-app/latest/tool-reference/analysis/summarize-within.htm

Calculating geometry - changing the units does NOT change the coordinate system used, just the default units for the coordinate system. You can specify the coordinate system used (to be different to the input data):

https://pro.arcgis.com/en/pro-app/latest/tool-reference/data-management/calculate-geometry-attribute...

Please don't forget the kudos 🙂