QGIS Elevation Units – What Are the X Units for an Elevation Profile and How to Change Them to Meters

elevationqgisqgis-3units

The question: How do I get a simple elevation profile for a line using QGIS 3.4 with height/distance in either meters or feet?

The problem: I have the same problem using both Profile Tool and qProf plugins: I get an elevation profile where the y axis is correctly in meters, and the x axis units are ???. In real life, we estimate the length of the line should be about 1/4 mile (or ~400 meters), but x coordinates read: 0.001, 0.002, up to nearly 0.005. (am I off a factor of 10,0000?)

I have spent quite a long time searching the internet for similar questions, but haven’t found the solution to my problem. Here’s a detailed description of how I got here:

1. Download Digital Elevation Model (DEM) file:

https://www.usgs.gov/core-science-systems/ngp/tnm-delivery/

  • select Data Download Application: Launch

https://viewer.nationalmap.gov/basic/

  • select Elevation Products (3DEP)
  • select 1/3 arc-second DEM
  • locate an area on the map on the right
  • I chose: Use Map and Current Extent, then Find Products
  • I chose and downloaded: USGS NED 1/3 arc-second n42w122 1 x 1 degree
    ArcGrid 2019

2. Understand CoordinateReferenceSystem (CRS) of DEM file

Unzip n42w123.zip, and load hdr.adf into the qgis project (via select/drag from browser window). (I have also tried adding a raster layer and choosing the w001001.adf file)

Clicking the layer properties and information tab, I see that:

  • the CRS is EPSG:4269 - NAD83 - Geographic
  • the unit is in degrees
  • and I confirmed from a metafile somewhere that the height z axis is in meters

3. Change the CRS of the DEM layer

All the answers on the internet describe that you need to project to a flat UTM coordinate system for slope and elevation profile stuff to work. I’ve tried several:

  • right click on the DEM layer
  • Set CRS
  • Set Layer CRS
  • Choose NAD83 / California Zone 1 EPSG:26941 (I’ve also tried NAD83/California Zone 1 (ftUS) EPSG 2225 and NAD83/UTM zone 10N).
  • view the layer properties again and confirm that units are now in meters

4. Load the line and convert CRS
I had a kml file that I loaded and then exported to a shape file. Loading that shape file and viewing the layer properties, I see the default:

  • CRS EPSG:4326 – WGS 84 – Geographic
  • Geometry Line (MultiLineString)
  • Unit degrees
  • right click the layer and Set Layer CRS to match above: NAD83 / California Zone 1 EPSG:26941
  • review layer properties and confirm: crs has changed and unit is now meters.

5. Download QGIS plugins:
–

  • go to Plugins, then Manage and Install Plugins, and download Profile Tool and qProf

6. Profile Tool

  • select Plugins, Profile Tool, Terrain Profile
  • highlight DEM layer, then Add Layer button in Profile Tool
  • highlight line layer, then choose Options Selection Selected layer in Profile Tool
  • In the Profile view, I have an elevation profile where the Y axis looks to be correct elevation in meters, but the X axis has units 0, .001, .002… to just under maybe .006. WHAT IS THIS? It can’t be meters or kilometers — it doesn’t line up.
  • Instead of selecting a line layer, choose Options Selection Temporary poly line, then draw a line. Same X-axis units

7. qProf

  • select Plugins, qProf, qProf
  • DEM input: choose Define source DEMs and select DEM layer
  • select Line layer, choose layer, and select line layer
  • leave line densify distance at 9.25..e-05 ?
  • Read source data
  • Calculate profile statistics (notably, it says Profile length: 0.004851)
  • Create topographic profile, accept defaults, OK
  • chart goes from 0 to just under .005 (so same results as Profile tool)

8. Other notes:

I changed the layers and the project CRS to EPSG:26910 - NAD83 / UTM zone 10N and got the same results

When I switch to NAD83/California Zone 1 (ftUS) EPSG 2225 CRS
I get a Profile length: 0.001479
(if I multiply that by 10,000, I get 1479 feet which is .28 mile which is roughly what I expect, but I don’t know if there’s a basis for the correlation)
But I notice the height (y axis) remains in meters, so it’s better not to use this projection.

Changing the project general properties to measure distance in meters (or something else) has no effect on the profile tool.

9. Repeat the question:
What are the X units in the QGIS Profile Tool? How can I change them to meters, kilometers, or miles?

Best Answer

Step 3 is the correct attempt. But instead of "Set CRS" choose "Export as".

"Set CRS" does just define your CRS without actually changing it. It still is the same as before, but now you only "tell" it is different, while it actually isnt.

"Export as" does really reproject your data.

So do it the following: "Set your CRS" back to its original CRS. Then "Export it as" a new file. Here you can choose a projected CRS with meters as unit.

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