Same Projection Produces Different Measurement Results In ArcMap/Web Map

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10-05-2020 10:39 AM
RyanUthoff
Occasional Contributor III

I am noticing a difference in measurements produced in ArcMap/Pro vs. the exact same dataset in both Portal and AGOL.

I have a polygon dataset in the Web Mercator coordinate system. The data frame of ArcMap/Pro is also set to Web Mercator. If I measure the width of the polygon in ArcMap/Pro, it is 210ft. However, when I publish this dataset to both server and AGOL, the width of the polygon changes to 141ft. 

I understand that server automatically reprojects data to the Web Mercator coordinate system. However, in this case, the dataset is already in the Web Mercator coordinate system, so there is nothing to reproject. So why am I getting different results when using the measurement tool?

What is strange is when I have a dataset in a different coordinate system other than Web Mercator and publish it to server, the measurements are correct and match what is in ArcMap/Pro. Both in ArcMap and Portal, the width is 141ft. That is where I would have thought the measurements would be different since the server is automatically reprojecting the data to Web Mercator. This leads me to believe ArcMap isn't projecting the data properly in Web Mercator since that is the only place where I get a measurement that is way off. Any suggestions?

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RachelSears
Occasional Contributor II

Hi Ryan,

Thank you for the clarification. I did some investigation and I think it could have to do with the different measurement settings. Which measurement type are you using in ArcMap? I believe that by default it uses the "Planar" measurement setting, which would be appropriate for a 2D projected coordinate system, but WGS 1984 Web Mercator is based on a spheroid; therefore, the "Geodesic" measurement setting would be more appropriate. I believe this is the type of measurement used in the web map viewer.

Measuring distances and areas—ArcMap | Documentation 

Let me know if you are still seeing the discrepancy after changing the measurement type to "Geodesic" in ArcMap.

Best,

Rachel

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RachelSears
Occasional Contributor II

Hi Ryan,

When data is reprojected, there is always going to be some loss of accuracy. Some transformations have a more significant loss than others. Do you have any more information about how the original dataset was created? Was it always in WGS 1984 Web Mercator, or was it created in a different coordinate system and reprojected in ArcMap/ArcGIS Pro? Check the transformation of the data frame in the Properties under Coordinate Systems > Transformations.

This documentation might also be helpful:

Choosing an appropriate transformation—ArcMap | Documentation 

How To: Determine which NAD_1983_To_WGS_1984 transformation to use 

You might also want to check to see how the data is transformed within ArcGIS Pro to see if it is treating the data differently.

- Rachel

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RyanUthoff
Occasional Contributor III

Hi Rachel,

Thank you for replying back. I understand that reprojecting data can cause accuracy loss. However, in this case, I can replicate this among multiple datasets, even datasets I create in Web Mercator. So even in cases where the original dataset that I create is in Web Mercator, I am still seeing a significant size difference between what ArcMap is displaying vs. what the web maps are displaying. I would expect this if the server was reprojecting the data into another coordinate system, but in this case, the data is already in Web Mercator so I'm not sure why there is such a large difference in size.

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RachelSears
Occasional Contributor II

Hi Ryan,

Thank you for the clarification. I did some investigation and I think it could have to do with the different measurement settings. Which measurement type are you using in ArcMap? I believe that by default it uses the "Planar" measurement setting, which would be appropriate for a 2D projected coordinate system, but WGS 1984 Web Mercator is based on a spheroid; therefore, the "Geodesic" measurement setting would be more appropriate. I believe this is the type of measurement used in the web map viewer.

Measuring distances and areas—ArcMap | Documentation 

Let me know if you are still seeing the discrepancy after changing the measurement type to "Geodesic" in ArcMap.

Best,

Rachel

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RyanUthoff
Occasional Contributor III

Oh my goodness, that is exactly what it is! Once I change it to Geodesic in ArcMap, the measurements between ArcMap/Pro and the web map match up. I've been trying to figure out why the measurements didn't match for awhile now, and it was just a little setting in the measurement tool. Thank you for your help!

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