topic Re: Units when adding surface elevation to polylines in Elevation Data Questions

Re: Units when adding surface elevation to polylines

Cathina_Gunn_de_Rosas — Tue, 12 May 2015 15:21:09 GMT

I needed slope information for some polyline sections, and after adding surface information to the file and calculating total slope, I find that the result does not seem to match what was previously measures at one of the sites (my test section).

Now, I know the z values are in meters. They come from the National Map Project DEM file I used for the surface data. I would assume the "Shape_Le_1" is as well ... but that doesn't make sense when I convert to feet and take the slope. Where would ArcMap source those units from?

The overall data frame units for the layer are decimal degrees ... but the length can't be in decimal degrees because the values would be tiny.

I tried changing the data frame units to "feet" but still have the same values when I add surface data.

How can I be sure where these units are sourced from and what they are?

(I am a novice user so this is probably a ridiculous question but I have to be 100% about the units)

Re: Units when adding surface elevation to polylines

SepheFox — Tue, 12 May 2015 15:33:44 GMT

Hi Cathina, to find the linear units of any layer, right click on it, select properties, and then select the source tab. Linear unit is different from angular unit, such as degrees. How exactly are you doing the conversion to feet? It's probably a good ide to get your DEM and polylines into the same projection before you run the analysis, by using either the Project or Project Raster tool. Be aware though, that you may never get a perfect match between the ground measurement, and your calculated one , due to the resolution and accuracy of your DEM.

Re: Units when adding surface elevation to polylines

Cathina_Gunn_de_Rosas — Tue, 12 May 2015 15:51:43 GMT

Well, it looks as though the polylines are in meters, but the "linear units" section for the DEM is empty (which makes sense as it is elevation/z data).

In this regime, would the "shape length" be in meters (from the polyline) and the z values be in meters (from the DEM)?

They are both from the NAD 1983 datum / source

Re: Units when adding surface elevation to polylines

Cathina_Gunn_de_Rosas — Tue, 12 May 2015 15:55:19 GMT

(also, my simple conversion from meters to feet is ~ X m * (0.03048 ft/1m) )

Re: Units when adding surface elevation to polylines

SepheFox — Tue, 12 May 2015 15:57:42 GMT

Hmmm, if it has a spatial reference it should still have a linear unit, regardless of what the data represents, because it still has x and y values. See this screenshot for a DEM I have:

What does your DEM say for spatial reference?

Re: Units when adding surface elevation to polylines

Cathina_Gunn_de_Rosas — Tue, 12 May 2015 16:20:54 GMT

Oh, that is strange. It says GCS_North_American_1983

Here is a screenshot ... I hate that this is such a mystery even at this stage. I have a whole new question about the calculated slope to post, but I'm not going there until I get this unit thing worked out.

Re: Units when adding surface elevation to polylines

SepheFox — Tue, 12 May 2015 16:27:11 GMT

Ok, what does your original polyline layer say when you do the same?

Re: Units when adding surface elevation to polylines

Cathina_Gunn_de_Rosas — Tue, 12 May 2015 16:35:59 GMT

That is the NAD 1983 ... here

Re: Units when adding surface elevation to polylines

SepheFox — Tue, 12 May 2015 16:43:50 GMT

Ok, you need to use the Project Raster tool (ArcGIS Desktop ), to reproject your raster into the exact same projection as the polyline.

Re: Units when adding surface elevation to polylines

Cathina_Gunn_de_Rosas — Tue, 12 May 2015 17:11:18 GMT

Thank you - I really appreciate the help!

I got them projected on the same datum and the linear units are correct now.

Now on to the slope porblems ...