Doc: Add DTM tutorial

doc/source/glossary.rst

@@ -68,6 +68,11 @@ The GDAL glossary contains terms and acronyms found throughout the GDAL document
 
         A software component that enables reading, writing, and processing of specific raster or vector data formats.
 
+    DTM
+
+        Digital Terrain Model. A raster representation of the bare ground surface of the earth, excluding natural and man-made
+        objects such as vegetation and buildings.
+
     Ellipsoid
 
         A model used to approximate the Earth's shape in coordinate systems.
@@ -98,6 +103,14 @@ The GDAL glossary contains terms and acronyms found throughout the GDAL document
 
         An identifier used by an operating system to manage and access an open file during a program's execution.
 
+    GEOS
+
+        Geometry Engine - Open Source. GEOS is a C/C++ library for computational geometry with a focus on algorithms
+        used in geographic information systems (GIS) software. GEOS started as a port of the Java Topology Suite (JTS).
+
+        .. seealso::
+            https://libgeos.org/
+
     Gridding
 
         The process of converting scattered or irregularly spaced data points into a regular, structured grid format.
@@ -175,6 +188,14 @@ The GDAL glossary contains terms and acronyms found throughout the GDAL document
 
         A method that finds the closest data point(s) to a given point, often used for classification or estimation based on similarity.
 
+    OGC
+
+        Open Geospatial Consortium. An international, non-profit organization that develops and promotes open standards
+        for geospatial data and services.
+
+        .. seealso::
+            https://www.ogc.org/
+
     OSR
 
         OGR Spatial Reference (OSR) - module that handles spatial reference systems and coordinate transformations.
@@ -293,6 +314,13 @@ The GDAL glossary contains terms and acronyms found throughout the GDAL document
         .. seealso::
             :ref:`gdalwarp`.
 
+    WFS
+
+        Web Feature Service, an :term:`OGC` standard that allows users to access geospatial vector data over the web.
+
+        .. seealso::
+            :ref:`vector.wfs`
+
     WKT
 
         Well-Known Text. Text representation of geometries described in the Simple Features for SQL (SFSQL) specification.
@@ -307,6 +335,13 @@ The GDAL glossary contains terms and acronyms found throughout the GDAL document
 
         Well-Known Binary. Binary representation of geometries described in the Simple Features for SQL (SFSQL) specification.
 
+    WMS
+
+        Web Map Service, an :term:`OGC` standard that allows users to request and display georeferenced map images over the web.
+
+        .. seealso::
+            :ref:`raster.wms`
+
 
 Credits and Acknowledgments
 ---------------------------

doc/source/tutorials/code/color-map-percentage.txt

@@ -0,0 +1,12 @@
+nv 255 255 255 0
+0% 112 147 141
+10% 120 159 152
+20% 130 165 159
+30% 145 177 171
+40% 180 192 180
+50% 212 201 180
+60% 212 184 163
+70% 212 193 179
+80% 212 207 204
+90% 220 220 220
+100% 235 235 237

doc/source/tutorials/index.rst

@@ -15,6 +15,7 @@ Raster
    warp_tut
    gdal_grid_tut
    geotransforms_tut
+   raster_dtm_tut
 
 Multidimensional raster
 -----------------------

doc/source/tutorials/raster_dtm_tut.rst

@@ -0,0 +1,272 @@
+.. _dtm_tut:
+
+================================================================================
+Working with a Digital Terrain Model (DTM)
+================================================================================
+
+.. contents::
+    :depth: 3
+
+Overview
+--------
+
+This tutorial demonstrates how to work with a :term:`DTM` (Digital Terrain Model), using freely available datasets.
+You'll learn how to create a raster tile index, clip data to a polygon, and generate hillshade, aspect, and slope images with various GDAL commands.
+
+Code examples assume a Bash shell on Linux. Where syntax differs, equivalent PowerShell commands are provided for Windows users.
+The tutorial covers both the unified :ref:`gdal command-line interface <programs_gdal>` and the :ref:`traditional <programs_traditional>`
+individual GDAL tools.
+
+Setting Up a Conda Environment
+------------------------------
+
+This tutorial assumes Conda is already installed. We'll start by creating a new Conda environment and installing GDAL.
+
+.. code-block:: console
+
+    $ conda create --yes --name gdal
+    $ conda activate gdal
+    $ conda install --yes -c conda-forge gdal
+    $ conda --version
+    conda 24.3.0
+    $ gdal --version
+    GDAL 3.11.3 "Eganville", released 2025/07/12
+
+Getting the Data
+----------------
+
+We'll use open data from IGN (Institut national de l'information géographique et forestière), France's national mapping agency.
+IGN provides a national DTM (the RGEALTI dataset) available at https://geoservices.ign.fr/rgealti.
+
+This tutorial focuses on department 90 - "Territoire de Belfort" (a department is an administrative unit in France).
+The DTM for this area is available as a ``.7z`` archive: `RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13.7z`_ (45 MB).
+You can automate the download and extraction using the commands below.
+
+.. tabs::
+
+   .. code-tab:: bash
+
+        # ensure 7-zip is installed
+        apt-get update && apt-get install curl p7zip-full -y
+
+        # use curl to download the file
+        curl -O https://data.geopf.fr/telechargement/download/RGEALTI/RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13/RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13.7z
+
+        # unzip the contents
+        7z x RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13.7z
+
+   .. code-tab:: ps1
+
+        # hiding progress output significantly speeds up downloading files
+        $ProgressPreference = "SilentlyContinue"
+
+        # download the zip file
+        Invoke-WebRequest `
+          -Uri "https://data.geopf.fr/telechargement/download/RGEALTI/RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13/RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13.7z" `
+          -OutFile "RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13.7z"
+
+        # download the 7-zip installer
+        Invoke-WebRequest -Uri "https://www.7-zip.com/a/7z2409-x64.exe" -OutFile "7z-x64.exe"
+
+        # unzip the contents
+        Start-Process -FilePath "7z-x64.exe" -ArgumentList "/S" -Wait
+        & "C:\Program Files\7-Zip\7z.exe" x "RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13.7z" -y
+
+Creating a Raster Tile Index
+----------------------------
+
+The archive contains individual tiles in the ASCII raster (``.asc``) format - a plain text format commonly used for gridded spatial data.
+
+.. image:: ../../images/tutorials/asc-files.png
+
+Rather than working with each file separately, we can treat them as a single virtual raster dataset using the :ref:`GDAL Raster Tile Index <raster.gti>` driver.
+The tile index can be created using the new CLI tool :ref:`gdal_driver_gti_create` or with :ref:`gdaltindex` from the traditional GDAL tools.
+This creates a vector dataset with one record per raster tile, allowing the tiles to be accessed as a unified raster in GDAL, MapServer, or QGIS (version 3.4+).
+
+.. tabs::
+
+   .. code-tab:: bash gdal CLI
+
+        files="RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13/RGEALTI/1_DONNEES_LIVRAISON_2021-10-00009/RGEALTI_MNT_5M_ASC_LAMB93_IGN69_D090/*.asc"
+        gdal driver gti create --layer dtm --dst-crs EPSG:2154 --output-format FlatGeobuf $files tileindex.gti.fgb
+
+   .. code-tab:: bash Traditional
+
+        files="RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13/RGEALTI/1_DONNEES_LIVRAISON_2021-10-00009/RGEALTI_MNT_5M_ASC_LAMB93_IGN69_D090/*.asc"
+        gdaltindex -lyr_name dtm -t_srs EPSG:2154 -f FlatGeobuf tileindex.gti.fgb $files
+
+   .. code-tab:: powershell
+
+        # create a GTI from all the asc files
+        $files = "RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13/RGEALTI/1_DONNEES_LIVRAISON_2021-10-00009/RGEALTI_MNT_5M_ASC_LAMB93_IGN69_D090/*.asc"
+        gdal driver gti create --layer dtm --dst-crs EPSG:2154 --output-format FlatGeobuf $files tileindex.gti.fgb
+
+Clipping the DTM
+----------------
+
+Next, we'll clip the DTM using a polygon representing department 90. Instead of downloading a national dataset of all departments, we can use a WFS server
+to retrieve only the feature we need.
+
+For the traditional approach we'll use :ref:`ogr2ogr` and the :ref:`vector.wfs` driver.
+Starting with GDAL 3.11, the same result can be achieved using the GDAL CLI and the :ref:`gdal_vector_pipeline`.
+
+In both cases, we explicitly set the output layer name to *commune* to avoid the auto-generated name *ADMINEXPRESS-COG.2017:commune*.
+When using ``ogr2ogr`` it's also important to set the geometry type with ``-nlt POLYGON``, otherwise, clipping the raster will fail with the error:
+``ERROR 1: Cutline not of polygon type``.
+
+.. tabs::
+
+   .. code-tab:: bash gdal CLI
+
+        gdal vector pipeline \
+            ! read WFS:"https://data.geopf.fr/wfs/ows?version=2.0.0&typename=ADMINEXPRESS-COG.2017:commune" \
+            ! filter --where "insee_com='90065'" \
+            ! write --output-layer=commune commune.fgb
+
+   .. code-tab:: bash Traditional
+
+        ogr2ogr -f FlatGeobuf commune.fgb \
+            WFS:"https://data.geopf.fr/wfs/ows?version=2.0.0&typename=ADMINEXPRESS-COG.2017:commune" \
+            -nln commune \
+            -nlt POLYGON \
+            -where "insee_com = '90065'"
+
+   .. code-tab:: powershell
+
+        gdal vector pipeline `
+            ! read WFS:"https://data.geopf.fr/wfs/ows?version=2.0.0&typename=ADMINEXPRESS-COG.2017:commune" `
+            ! filter --where "insee_com='90065'" `
+            ! write --output-layer=commune commune.fgb
+
+The image below shows the polygon downloaded from the WFS server overlaid on the raster tile index. We will
+clip the tiled raster using this polygon to create a new  **clipped.tif** file using :ref:`gdal_raster_clip` for the CLI approach,
+and :ref:`gdalwarp` for the traditional method. 
+
+.. image:: ../../images/tutorials/clip.png
+
+GDAL can clip the raster dataset even though the polygon is in a different projection. However when using the CLI you must add
+``--allow-bbox-outside-source`` to avoid the following error caused by the projection mismatch:
+
+``ERROR 1: clip: Clipping geometry is partially or totally outside the extent of the raster.
+You can set the 'allow-bbox-outside-source' argument to proceed.``
+
+.. tabs::
+
+   .. code-tab:: bash gdal CLI
+
+        gdal raster clip --like commune.fgb  tileindex.gti.fgb clipped.tif --overwrite --allow-bbox-outside-source
+
+   .. code-tab:: bash Traditional
+
+        gdalwarp -cutline commune.fgb -crop_to_cutline -of GTiff tileindex.gti.fgb clipped.tif
+
+.. note::
+
+    To run spatial operations like clipping, your GDAL installation must include the :term:`GEOS` dependency. On UNIX systems
+    you can verify this using the :ref:`gdal-config` program (not available on Windows).
+
+    .. code-block:: bash
+
+        gdal-config --dep-libs
+        # the output should include -lgeos_c
+
+    If GEOS is not installed, you may encounter errors like the following:
+
+    .. code-block:: bash
+
+        # ERROR 6: GEOS support not enabled.
+
+Processing the DTM
+------------------
+
+With a raster covering our area of interest, we can now use GDAL's powerful raster tools.
+For an excellent overview of creating relief maps with GDAL, see
+`A Gentle Introduction to GDAL Part 5: Shaded Relief <https://medium.com/@robsimmon/a-gentle-introduction-to-gdal-part-5-shaded-relief-ec29601db654>`__.
+
+The traditional tools for analyzing and visualizing :term:`DEM` s and :term:`DTM` s are the :ref:`gdaldem` commands. The new CLI provides similar functionality
+under the ``gdal raster`` subcommands.
+
+First we'll create a shaded relief map using :ref:`gdaldem` and ``hillshade`` (traditional approach) or :ref:`gdal_raster_hillshade` with the GDAL CLI.
+Both commands support various parameters, such as light azimuth, altitude, and vertical exaggeration.
+Refer to the command help pages for full details, and feel free to experiment with the examples below.
+
+.. tabs::
+
+   .. code-tab:: bash gdal CLI
+
+        gdal raster hillshade --zfactor=5 clipped.tif hillshade.tif --overwrite
+
+   .. code-tab:: bash Traditional
+
+        gdaldem hillshade -z 5 clipped.tif hillshade.tif
+
+The output of this command is shown below. Note that images have been converted to the PNG format and resized for the GDAL documentation.
+
+.. image:: ../../images/tutorials/hillshade.png
+
+Next, we'll create a slope map using :ref:`gdaldem` and ``slope`` (traditional approach), or :ref:`gdal_raster_slope` with the GDAL CLI.
+Feel free to experiment with the available settings to produce a visualization that best represents the data.
+
+.. tabs::
+
+   .. code-tab:: bash gdal CLI
+
+        gdal raster slope clipped.tif slope.tif --overwrite
+
+   .. code-tab:: bash Traditional
+
+        gdaldem slope clipped.tif slope.tif
+
+Now we'll create an image showing terrain aspect using :ref:`gdaldem` and ``aspect`` (traditional approach) or :ref:`gdal_raster_aspect` with the GDAL CLI.
+
+.. tabs::
+
+   .. code-tab:: bash gdal CLI
+
+        gdal raster aspect clipped.tif aspect.tif --overwrite
+
+   .. code-tab:: bash Traditional
+
+        gdaldem aspect clipped.tif aspect.tif
+
+
+.. image:: ../../images/tutorials/aspect.png
+
+Next, we'll apply color to the DTM based on elevation values. First, create a file named ``color-map-percentage.txt`` in the same folder as your ``clipped.tif`` file,
+and copy in the contents below. The file defines :term:`RGB` values for elevation ranges, grouped by percentage. GDAL maps these percentage
+ranges to RGB colors when rendering the DTM. The ``nv`` value specifies the RGB colour for any ``NoData`` values.
+
+.. literalinclude:: code/color-map-percentage.txt
+
+Now we can apply the color map to the DTM using :ref:`gdaldem` and ``color-relief`` (traditional approach) or :ref:`gdal_raster_color_map` with the GDAL CLI.
+
+.. tabs::
+
+   .. code-tab:: bash gdal CLI
+
+        gdal raster color-map --color-map=color-map-percentage.txt clipped.tif color.tif --overwrite
+
+   .. code-tab:: bash Traditional
+
+        gdaldem color-relief clipped.tif color-map-percentage.txt color.tif
+
+.. image:: ../../images/tutorials/color.png
+
+To finish, we'll use a new tool introduced in GDAL 3.12  - :ref:`gdal_raster_color_merge`  to blend the color map with the previously created hillshade image.
+This command is available only through the new GDAL CLI.
+
+.. code-block:: bash
+
+    gdal raster color-merge --grayscale=hillshade.tif color.tif color-hillshade.jpg
+
+.. image:: ../../images/tutorials/color-hillshade.jpg
+
+.. _`RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13.7z`: https://data.geopf.fr/telechargement/download/RGEALTI/RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13/RGEALTI_2-0_5M_ASC_LAMB93-IGN69_D090_2021-01-13.7z
+
+.. spelling:word-list::
+    Institut
+    l'information
+    géographique
+    forestière
+    Territoire
+    Belfort