Clarification regarding planar, geodesic and preserveShape calculation types

[SarshadAbubaker]

Hi there,

I'm having a bit of a confusion and I'm really hoping someone can clear this out. I'm aware of the three calculation modes for the geometry service "lengths" and "areasAndLengths" operations but am slightly confused about the following:

1) If you're using preserveShape or geodesic calculation types, do you need to project to the spatial reference of the area in which you are performing your measurements for accurate results? I seem to be getting the same (correct) result whether or not I project from Web Mercator (102100) to WKID 2229 for LA County. Or is a projection to the correct coordinate system only needed for the planar calculation type?

2) If I'm using the correct projection, will a planar calculation type give me the same (correct) results for measurements as "preserveShape"? This seems to be the case.

3) Will projecting before computing lengths or areas using "preserveShape" increase its accuracy in any way?

I'm using an ArcGIS 10.1+ geometry service and "preserveShape" is supported.

Any help or input would be much appreciated. I'm looked around but never got a satisfactory answer anywahere else. Thanks in advance.

Sarshad

[JianHuang · Accepted solution]

1. No, you don't have to reproject it to other projection. Regardless the current coordinate system, when choosing geodesic, it is truly geodesic result.
2. Yes, you can get very close result. But you cannot get the exact same result. Because there is no such a coordinate system to truly match planar and geodesic at the same time.
3. No.

[SarshadAbubaker]

Thanks a ton Jian. Have a great day!