#1 Importing elevation line as DEM in CRP. von Droniste 02.02.2021 00:52

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Digital Elevation Model get more accessible and open-source. It's pretty "easy" to produce elevation contours using a software like QGIS. It would be great to be able to import contour file to produce a pseudo DEM in CRP.
I have attached a KML format export file as an exemple if you want to have a look and see what could be done. There are many other formats available.[[File:contours_test_2.kml.zip]]

#2 RE: Importing elevation line as DEM in CRP. von CaveRenderPro 06.02.2021 16:31

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This example has 250 isohypses with at least 50000 points. Every point has to converted from WSG to UTM-Grid. This is probably very slow. Is the DEM available in UTM?

#3 RE: Importing elevation line as DEM in CRP. von Droniste 10.02.2021 00:06

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I believe WGS84 is standard to kml format and whatever coordinates datum is used, QGIS will converted them during the export to kml... I created another set of isohypses using GML format with UTM 31 coordinates. File attached below.

#4 RE: Importing elevation line as DEM in CRP. von CaveRenderPro 14.02.2021 14:31

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The gml file looks good.

I have installed QGIS today. Could you explain the process to load DEM data of a specific location and export them as isohypses to a glm file?

#5 RE: Importing elevation line as DEM in CRP. von Droniste 16.02.2021 14:32

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I assume you've already downloaded one or several DEM raster tiles and you know their original Coordinates Reference System (CRS).

Two sources of free/global DEM
NASA SRTM: https://earthexplorer.usgs.gov/
ALOS AW3D30: https://www.eorc.jaxa.jp/ALOS/en/aw3d30/data/index.htm

In QGIS:

Set the project Coordinates Reference System (CRS):
1 - Top menu bar: Projet > Properties > CRS > Select the appropriate UTM CRS.

Import DEM tiles into project:
2 - Top menu bar: Layer > add layer > add raster layer >> select raster dataset(s) > Open.
3 - Set DEM tiles' CRS: in Layers panel: right clic on each tile > Layers CRS > Select the appropriate original DEM CRS (potentially different from UTM)

If several adjacent tiles, create one Virtual Raster:
4 - top menu bar > Raster > Miscellaneous > Build Virtual Raster >> Select input tiles, Resolution "High", untick "place each input file into separate band", in "Virtual": save to file > Run.
5 - Set DEM virtual tiles' CRS: in Layers panel: right clic on virtual tile > Layers CRS > Select the appropriate original DEM CRS (potentially different from UTM)

Create contours from DEM tile or from virtual DEM tile:
6 - top menu bar: Raster > Extraction > Contours >> select input layer, define interval between contour lines, save to file > Run.
7 - Set DEM contours layer's CRS: in Layers panel: right clic on contours layer > Layers CRS > Select the appropriate contours CRS (should be the same as original DEM)

Export contours to GML:
8 - in layer panels: right click on contours layer > Export > Save features as >> select Format = GML, set file name, set layer name, select export CRS: should be the appropriate UTM CRS > OK.

Done.

#6 RE: Importing elevation line as DEM in CRP. von CaveRenderPro 27.02.2021 18:51

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I have downloaded the GMTED2010 from USGS. It shows half of europe in 7,5 arcsec resolution.

Because I have no experiance in GIS, I don't know, if this is suitable to create a contur. I think that the area is to large an the resolution to low. Maybe you have some hints.

Our expeditions are mainly in this area: 47°31' - 47°34' N, 12°35' - 12°41' E

#7 RE: Importing elevation line as DEM in CRP. von Droniste 27.02.2021 21:09

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Hi Jochen,

That's a massive area and 7.5 arcsec resolution isn't great. In many countries, national DEM have become open-source, and they often have a high resolution and accuracy. You just need to know where to find them... If not, NASA SRTM (available on https://earthexplorer.usgs.gov/ in Dataset > Digital Elevation (where you found GMTED2010) > SRTM) and ALOS AW3D30 are a good source of free and global DEM. They both have a 30-metre resolution. AW3D30 claims a vertical accuracy of 5m, SRTM claims an absolute accuracy of 16m and relative accuracy of 6m (from pixel to neighboring pixels).

I had a look at the DEM available in your area: SRTM has unfortunately many voids (missing pixels/data) in your expedition area and won't be of much use. Luckily, the ALOS AW3D30 seems of a much better quality in this area : https://www.eorc.jaxa.jp/ALOS/en/aw3d30/data/index.htm. I believe you need to create an account first. For your expedition area, you can download the following corresponding tile here: https://www.eorc.jaxa.jp/ALOS/en/aw3d30/...45E010_N047E012

1 - From the package, open the *DSM.tif file in QGIS.
2 - Make sure the CRS is WGS84 (EPSG4326).
3 - Create the contours for the entire area. In Top menu: Raster>Extraction>contours >> Select input layer (DEM), set the required interval >Run

That remains lots of data and a huge area, so you need to extract from the contours created the contours for your expedition area.

4 - In top menu: Layer >Create Layer> New Shapefile Layer >> give the new layer a name, select Geometry type = polygon > OK
5 - In Layers Panel: Right clic on the newly created layer > Toggle Editing. This layer is now editable.
6 - In top menu: Edit> Add Polygon feature. Start drawing the polygon of your expedition area. When done right clic>> OK
7 - In top menu: Vector > Geoprocessing Tools>Clip...>> Input layer is the contour layer, Overlay layer is your expedition area > Run
8 - In Layers Panel: Right clic on the clipped contours layer>Export>Save feature as >> Format GML, give the file a name, select the export UTM CRS >> OK

Done!! :)

#8 RE: Importing elevation line as DEM in CRP. von CaveRenderPro 02.03.2021 23:03

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Preview

#9 RE: Importing elevation line as DEM in CRP. von Droniste 02.03.2021 23:25

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Prima!

#10 RE: Importing elevation line as DEM in CRP. von CaveRenderPro 06.03.2021 08:13

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First contour in CaveRenderPro:



Performance:
- 50 m contour, Area 12x12 km, 100.000 lines, file size 3,3 MB
- Additional loading time (once): 175 ms
- Additional calculation and display time (per frame): 35 ms

This is a great feature for CaveRenderPro major release 8.0.0.

#11 RE: Importing elevation line as DEM in CRP. von Droniste 10.03.2021 13:05

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The results is really impressive! Thank a lot!

#12 RE: Importing elevation line as DEM in CRP. von Droniste 23.08.2021 14:49

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Hi Jochen,

Would it be possible to display the contours in Plan mode?

#13 RE: Importing elevation line as DEM in CRP. von CaveRenderPro 30.08.2021 23:14

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What is the "User Story"? A user uses this funcrion to ....

#14 RE: Importing elevation line as DEM in CRP. von Droniste 31.08.2021 10:46

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My user story... I'd like to be able to overlay surface contours with the cave, to highlight the relation between surface and cave(s) in plan mode: proximity with the surface or crossing through a mountain, valley and riverbeds... as well as the topographic relief for cave access.
Ideally it should be exportable as a drawing layer (like we can with surevy shoots).

#15 RE: Importing elevation line as DEM in CRP. von CaveRenderPro 29.09.2021 13:01

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This feature is not planned in short time.

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