topic Re: Measure dijit - Geodesic Vs Planar in ArcGIS JavaScript Maps SDK Questions

Measure dijit - Geodesic Vs Planar

ManojPavuluri — Fri, 07 Dec 2012 14:22:45 GMT

Hi all,

Is there a way to change the measure dijit to do the measurements using planar calculations instead of Geodesic.

Map is in web mercator aux sphere. From the documentation it says planar calculations are for projected coordinate systems. Why are the measurements in geodesic.

Any help or workarounds?

Thanks[ATTACH=CONFIG]19788[/ATTACH]

Manoj

Re: Measure dijit - Geodesic Vs Planar

JohnGravois — Fri, 07 Dec 2012 15:25:28 GMT

Planar measurements calculated in a map whose projection is Web Mercator would be grossly inaccurate because the projection itself distorts distances, so calculating geodesic measurements is far more appropriate.

see the following knowledge base article for more information
http://support.esri.com/en/knowledgebase/techarticles/detail/39404

It is certainly possible to calculate planar measurements using the areasAndLengths() method of the geometry service task, but this wouldn't be appropriate for a web application with a Web Mercator basemap.

Re: Measure dijit - Geodesic Vs Planar

ManojPavuluri — Fri, 07 Dec 2012 19:42:36 GMT

Thanks for the clarification.

We have services published with data that are in different states. So are the layers' projections, UTM Zone x, State Plane etc. We are using Web Mercator for publishing the services to use ESRI and Bing imagery as background.

Measuring areas or distances is off using the measurement dijit from the API. And the clients want the measurements to match with what they see in ArcMap. Is there any setting that can be set to calculate measurements in planar. Or do we need to implement a custom tool using the geomerty service to calculate areas and lengths.

Thanks.

Manoj

Re: Measure dijit - Geodesic Vs Planar

JohnGravois — Fri, 07 Dec 2012 20:24:27 GMT

in order to accomplish this you would have to...

1. reproject the web mercator coordinates to the appropriate coordinate system
2. measure the distance between the new coordinates using the planar calculationType

Re: Measure dijit - Geodesic Vs Planar

MarcoBoeringa — Sat, 08 Dec 2012 18:47:40 GMT

And the clients want the measurements to match with what they see in ArcMap. Is there any setting that can be set to calculate measurements in planar. Or do we need to implement a custom tool using the geometry service to calculate areas and lengths.

Thanks.

Manoj

If the client is doing planar calculations on this type of data/projection, the better practice might be to teach them to do geodesic calculations in ArcMap instead of trying to do inappropriate planar calculations on the Web Mercator web service. As John said, the measurements may be way off their true values if they client doesn't account for specific limitations of certain projection types. The only other alternative is as John says reprojecting the data to some sort of (local) projection that preserves distance relationships.