topic Projection for Terrain Slope Degrees Image Server Layer in ArcGIS Pro Questions

Projection for Terrain Slope Degrees Image Server Layer

AlexandraThompson90 — Tue, 29 Dec 2020 14:31:42 GMT

I want to use the "Terrain: Slope in Degrees" Image Service to Extract Values to Points for 4,000 locally-stored points across the continental US. Specifically, the points are in Kansas, Texas (& surrounding states), North Dakota, and Pennsylvania.

Here's the Image Server site: https://www.arcgis.com/home/item.html?id=af25a795273440deb449b336543602be

The site presents this warning:

WARNING: Slope is computed in the projection specified by the client software. The server resamples the elevation data to the requested projection and pixel size and then computes slope. Slope should be requested in a projection that maintains correct scale in x and y directions for the area of interest. Using geographic coordinates will give incorrect results. For the WGS84 Mercator and WGS Web Mercator (auxiliary sphere) projections used by many web applications, a correction factor has been included to correct for latitude-dependent scale changes.

I'm unclear on what an appropriate projection would be to "maintain correct scale in x and y directions" and whether this means to choose equidistant, equal area, etc. Would North America Equidistant Conic work?

edit:

I've tried "requesting" the image service in North America Equidistant Conic, and the results look the same as if I don't project it. In addition, requesting the layer in various cell sizes appears to make no difference on the resulting image service layer.

I don't need slope degrees; slope percent would be fine. Just want to heed the "warning" given.

Re: Projection for Terrain Slope Degrees Image Server Layer

DanPatterson — Mon, 28 Dec 2020 23:35:37 GMT

How Slope works—ArcGIS Pro | Documentation

There isn't a geodesic method online, if your coordinates are in decimal degrees?

Re: Projection for Terrain Slope Degrees Image Server Layer

AlexandraThompson90 — Tue, 29 Dec 2020 00:37:46 GMT

I'm not sure what you mean by "online." I'm working in Pro (specifically, everything's in python). Are you suggesting I create a new slope raster in AGO?

Re: Projection for Terrain Slope Degrees Image Server Layer

DanPatterson — Tue, 29 Dec 2020 01:17:52 GMT

You mentioned "image service"... I guessed you weren't working with locally stored data and/or trying to calculate slope on non-locally stored data.

If the data are local, the link may be appropriate then

Re: Projection for Terrain Slope Degrees Image Server Layer

AlexandraThompson90 — Tue, 29 Dec 2020 14:24:59 GMT

Oh I see. The image service is the "Terrain: Slope in Degrees" which I linked to. The points are locally stored and in geographic coordinates. The points are also proprietary and under contract agreements can't be uploaded to AGO.