ArcticDEM updates

[trey-stafford]

• [x] Review online-access options
• [ ] Update existing documentation for ArcticDEM addon
• [x] Create new README for pgc addons dir
• [x] Add new user reference on addons for QGreenland

[trey-stafford]

It does not look like we currently have any documentation on addons / ArcticDEM in RTD.

The existing/old website has this text on the downloads page, under Additional Data for QGreenland":

32-meter resolution ArcticDEM (digital elevation model): The Polar Geospatial Center has provided a custom Greenland 32-m resolution ArcticDEM for added detail. Download this add-on package and place it in a suitable location in your filesystem. Unzip the package and then follow the directions in the 'README.txt' file included in the package.

On the new website, on the "More Data" page, under "Custom QGreenland Resources":

32-meter resolution ArcticDEM (digital elevation model): The Polar Geospatial Center has provided a custom Greenland 32-m resolution ArcticDEM for added detail. Download this add-on package and place it in a suitable location in your filesystem. Unzip the package and then follow the directions in the 'README.txt' file included.

[trey-stafford]

Reviewing the new arctic dem v4.1 addon. It contains a README.txt that explains the layout of the data, how to access it from QGIS, how to build overviews, citation information, and how to style the data.

The section on styling the data is outdated. It references a 100m arctic dem in QGreenland, but we removed that in v2.0.0:

---

Styling this data:

If you would like the style of this 32m Arctic DEM to match the 100m Arctic DEM already in QGreenland, you can right-click on the Arctic DEM (100m) layer and select Styles > Copy Style and then right-click on the new 32m Arctic DEM layer and select ‘Styles > Paste Style’.

We still have the arctic_dem.qml file. We could bundle it with the addon. This just provides a hillshade view, which is fairly easy to do in the symbology menu already. One downside of hillshade symboloy is that it does not provide a color map/ramp for interpreting the data (the identify tool still works for inspecting individual values, however).

If we bundle the style with the data, we could look at adding it as a .qlr file and include additional metadata like the abstract/citation info as we normally do for QGreenland layers...

[trey-stafford]

PGC provided us with a link to an ArcGIS image server: https://overlord.pgc.umn.edu/arcgis/rest/services/elevation/pgc_arcticdem_mosaics_latest/ImageServer

This link can be used directly in QGIS's "ArcGIS REST Server" data source manager. It adds a WMS-like grayscale image with a colorbar/legend (without units). QGIS doesn't appear to allow the option to change the symbology.

A WMS endpoint is also available: https://overlord.pgc.umn.edu/arcgis/services/elevation/pgc_arcticdem_mosaics_latest/ImageServer/WMSServer?request=GetCapabilities&service=WMS

Using the WMS interface of the data source manager, I can add this URL and QGIS recognizes a layer that can be added. Note that the "PNG" image encoding option must be selected for the layer to work. This experience is similar to the ArcGIS REST Server option - a legend is provided for a grayscale image.

Finally, WCS is a listed option: https://overlord.pgc.umn.edu/arcgis/services/elevation/pgc_arcticdem_mosaics_latest/ImageServer/WCSServer

Connecting to the WCS service in QGIS shows an available layer named "1". Attempting to add this layer results in an error/warning icon next to the layer indicating that the data source is unavailable.

[MattF-NSIDC]

If we bundle the style with the data, we could look at adding it as a .qlr file and include additional metadata like the abstract/citation info as we normally do for QGreenland layers...

Exploring what can be done with QLR files feels like it will be valuable research for QGreenland-Net.

[trey-stafford]

Some replacement text for the website:

32-meter resolution ArcticDEM (digital elevation model): The Polar Geospatial
Center has provided a custom Greenland 32-m resolution ArcticDEM for added
detail. Two download packages are provided for the v4.1 ArcitcDEM:

* arcticdem_v4.1_32m_greenland_dem_only.zip: A ~4.1GB package containing only
  the 32-m resolution DEM.

* arcticdem_v4.1_32m_greenland_dem_full.zip: A ~9.5GB package containing the
  32-m resolution DEM and ancillary datasets (e.g., a "count" dataset giving the
  number of contributing DEMs for each pixel).

Download one of these add-on packages and place it in a suitable location in your
filesystem. Unzip the package and then follow the directions in the 'README.txt'
file included.

[trey-stafford]

Exploring what can be done with QLR files feels like it will be valuable research for QGreenland-Net.

Agreed. I tried exporting the layer w/ a manually edited abstract as a .qlr. That seemed to work OK! Still need to do some more testing to ensure we can e.g.., save w/ a relative path.

For now though, I think I'll update the "Styling" section of the package's README to point to QGIS resources:

-------------------
Styling this data:
-------------------

For information on changing the symbology of raster datasets in QGIS, see:

* QGIS lesson on changing raster symbology:
  https://docs.qgis.org/3.28/en/docs/training_manual/rasters/changing_symbology.html

* QGIS documentation on raster symbology properties:
  https://docs.qgis.org/3.28/en/docs/user_manual/working_with_raster/raster_properties.html#symbology-properties

[trey-stafford]

PR (draft) that leverages the ArcGIS image server to provide an online-access layer: https://github.com/nsidc/qgreenland/pull/799

[trey-stafford]

The PGC also provided us with some VRT files that reference data on AWS. I was not very successful with adding these to QGIS. Adding them as a layer causes QGIS to freeze.

799 looks like our best option atm to add ArcticDEM as an online-access layer.