Working with Trieste raster data with QGIS

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We already worked with some raster data in QGIS. We already georreferenced images and use them to create new features.

In this exercise, we will use some digital terrain models (DTM) to process raster data in QGIS.

For a brief introduction to these type of geographic information, you can follow the Ordenance Survey Raster and Vector data summary.

Download the DTM

Note: a copy of the downloaded tile is available from here. Use this copy for the exercices. Later you can explore NASA Earthdata Search website.

The ASTER GDEM data can be downloaded from the NASA Earthdata Search website. You need an account to download the data.

Draw a rectangle around Trieste and use ASTER as the search term. From the results, use the right button on the result to show the granules on the map.

Identify the granule (tile) to download. Then click on the download button to proceed.

Other source of DTM

http://www.pcn.minambiente.it/viewer/

Uncompress the tile

The data comes in a h5 file format, that QGIS can read.

Add the raster AG100.v003.46.013.0001.h5 to QGIS. Slect only the first layer ASTER_GDEM/ASTGDEM.

Reproject the DTM

We added two layer to QGIS. Each has a different coordinate system, and QGIS dynamically reprojects all layers to the map canvas coordinate system. In this case, since we add wuham.shp layer first, the map canvas is using the EPSG:4497 coordinate system, and the raster is reprojected on the fly.

Let's start by reprojecting the raster to the same EPSG:4497.

Raster reproject.png

Fill the reproject dialog with the following data:

Reproject dialog.png

You can remove the former ASTGTM2_N30E114_dem layer from the project.

Sampling our raster data

The original ASTGTM2_N30E114_dem.tif is an image with 3601x3601 pixels (our dem.tiff was rotated, and it became a 3451 by 3951 image). Each pixel has a value: its value is the mean height of that location.

The default style is based on a Singleband gray (as Render type), the gray does from Black to white (on Color gradient) and the values ranges from -0.474 to 118.996 meters. These estimated values, by doing a fast analysis if the raster. Remember that this image has 3601 x 3601 = 12967201, which are almost 13 million pixels.

To really know the pixel value range, we can:

  1. select Min/max
  2. select Actual (slower)
  3. Load button

New values are computed. You can compare the values before and after this sampling process.

As you can see, in this case we have values below 0 (we will find out where) and also values far more higher then the initial 118 estimation.

Styling

To style this raster more appropriately, we need:

  • Change the render type to Singleband pseudocolor
  • Toogle Invert (after the Spectral color map); blues for lower altitudes are reds for higher
  • We switch the Mode from Continuous to Equal interval
  • 6 classes, instead of 5
  • We set the Min elevation to 0 and the Max to 750
  • We use the Classify button to generate our 6 classes

Improved style based on intervals.png

With this new style (and removing the polygon fill from the vector layer) we get a new map like:

Map with improved style.png

Discussion

Values below 0 are not distinguished

Add transparency.png

Only a smaller are has higher values

Improved style based on intervals to 250.png

After these two adjustments, the map looks like:

Map with improved style adjusted.png

DEM analysis

Our raster dataset only has the elevation value for each pair of coordinates. We will use this precious information to compute:

  • Hillshade
  • Slope
  • Aspect

All are available from the DEM (Terrain models) menu entry.

DEM analysis menu.png

Hillshade

Fill the Output file with hillshade.tiff. Keep the default options.

Hillshade over pseudocolor:

In the following map, we use:

  • hillshade, with 60% transparency
  • pseudocolor band below

QGIS-ASTER-GDEM-hillshade.png

Slope

Fill the Output file with slope.tiff. Keep the default options.

You can use transparency to enhance the visual feedback.

In the following map, we use:

  • slope, with 80% transparency
  • hillshade, with 50% transparency

Slope-hillshade-dem.png

Aspect

Fill the Output file with aspect.tiff. Keep the default options.

Profile plugin

Profiletool-using-dem.png

Export the DEM

Install Qgis2threejs plugin in QGIS.

Repreoject the layers to EPSG:3045.

Export the project: Web → Qgis2threejs → Qgis2threejs

Export qgis2threejs default options.png

The result on the browser:

Trieste DEM with threejs default options.png