TerrainTool
The SRTM dataset used by TerrainTool has been moved (again!) by the US hosting team and now requires users to be logged in to an EROS account to access it. Whilst registering for an EROS account is pretty straightforward and free, this effectively forces users to use the tools provided by USGS/EROS to select and download the tiles they need. The immediate effect of this is that TerrainTool's auto-download feature no longer works, and, with regret, I have decided that it is not feasible to implement a stable solution when the host addresses and access protocols are subject to frequent change without notice.
In due course, I will release a new version of TerrainTool that has the auto-download feature tidily removed and will update the documentation including how to open an EROS account , which of their tools are recommended and where to place the data once it has been downloaded.
TerrainTool was written to create surface topographic data for the cave survey packages Survex and Therion using the results of the Shuttle Radar Topography Mission (SRTM) in which the shuttle Endeavour mapped the height of the Earth's surface between the latitudes 60 degrees North and 56 degrees South - about 80% of the Earth's land mass. Published resolution was 1 arc-second for the US and its territories and 3 arc-seconds elsewhere. The latter corresponds to about 90m at the equator. The resulting data is royalty-free and, for many countries, may be the only data publicly available. TerrainTool is able to automatically download the "tiles" of data that it needs, directly from a USGS server.
Because the SRTM radar imaging worked by using an oblique scan of the earth's surface, it had problems with shadowing where, despite multiple passes, areas remained hidden, resulting in voids in the data. This is particularly noticeable in mountainous areas. A subsequent project Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) used infra-red nadir-viewing camera (i.e. directly downward) which, in post-processing over multiple orbits created stereo images from which height could be derived. This overcame the shadowing problem but the view could be obstructed by cloud and vegetation cover and suffered from "artefacts" - spurious features which are by-products of the imaging process. On it's own, the ASTER Digital Elevation Model (DEM) is unsuitable for cave surveying.
Finally complete in February 2020, NASA released a new DEM by re-processing "global" 1 arc-second SRTM data and carefully filling in the voids with re-processed ASTER data. The result, NASADEM, is thus the highest resolution void-free near-global DEM that we have available. The only disadvantages are
- The tiles are typically 9 times the size of the 3 arc-second SRTM data, so they take longer to download and process and occupy more disk space
- You need to be logged into an Earthdata account to download the data. Unfortunately, TerrainTool cannot to do this for you so auto-download is disabled. However, signing up for an Earthdata account is quick and easy at https://urs.earthdata.nasa.gov/users/new/
With the completion of NASA DEM, this release of TerrainTool no longer directly supports the use of ASTER data since its use as a fill-in for voids in the SRTM data has already been done. TerrainTool now only offers a choice of SRTM (because it is small and automatically downloadable) and NASA (because of its higher quality and resolution)
Having logged in to your Earthdata account, you can find the NASADEM data by going to https://lpdaac.usgs.gov/products/nasadem_hgtv001/ ) selecting "Access Data" and "Download Data" next to "Data Pool" and clicking on the directory link will take you to a huge page containing all the tiles, currently at https://e4ftl01.cr.usgs.gov/MEASURES/NASADEM_HGT.001/2000.02.11/ Please note that this URL might be subject to change without notice. The page can take several minutes to download so don't panic if your browser freezes! Tile files are the ones ending ".zip" and preceded by the relevant lat/long e.g. NASADEM_HGT_n52w003.zip You can download tiles with a browser. Do NOT try unpacking the zip files - TerrainTool reads them in zipped form.
TerrainTool does the following:-
- If required, automatically downloads SRTM data from the USGS server.
- Converts between the spherical (Lat/Lon) coordinates used in the tiles and a variety of map coordinates used by cave surveyors so that the surface mesh and cave survey can be combined in a single model based on the local map coordinate system. Coordinate systems currently supported include British (OSGB) grid, Irish grid, UTM, French (Lambert conical projections), Austrian grids, Slovenian Grid and New Zealand's NZMG and NZTM2000. The internal design is intended to allow additional coordinate systems to be added easily.
- Re-samples the data using biaxial interpolation to create a rectangular mesh of user-specified spacing.
- Displays a coloured topographic map of the mesh.
- Adds a user-specified offset (3-D) to the coordinates to align with coordinates used for the underground survey.
- Saves the mesh as surface data in Survex (.svx) or Therion (.th) format.
The programme, written in Java, provides a conventional GUI-style interface and will run under Windows, Solaris, macOS and Linux operating systems. This version is built to run with JDK 15 which can be downloaded free of charge from https://jdk.java.net/15/ . Source code is compatible back to JDK 8 but the use of older Java versions is not recommended for security reasons.
"TerrainTool" was written by Mike McCombe who is very grateful to UBSS for giving it a home. Please feel free to contact Mike with feedback or requests for help at mike<at>mikemccombe.co.uk or via the Survex list.
Installation
Sorry, there's no fancy package installer but as there's only one file it should be pretty straightforward.
- If you don't already have Open JDK 15 installed, download it from the Open JDK website now. Windows users will need to set the system PATH variable to point to the JDK-15.0.1\bin directory and remove references to any earlier Java releases
- Download the TerrainTool "jar". Depending on your browser you may find that the file that arrives is called TerrainTool.zip. If this happens, just rename it to be TerrainTool.jar. Do not try unpacking the ZIP file!
- Double-clicking the jar file should start the programme. If you need to start it from a command line interface, try "java -jar TerrainTool.jar".
Thanks to Wookey, TerrainTool has been available in Debian and Ubuntu since 2012 (7/'Wheezy' and 12.04/'Precise Pangolin' respectively). To install it select 'terraintool' in your favourite package manager (Upgrade Center, Synaptic, Aptitude) and install it. On the command line do
sudo apt-get install terraintool
If your system has a firewall, you may need to tell it to allow java to have outgoing https access. Otherwise, it won't be able to download the SRTM data files from the USGS site. This version of TerrainTool doesn't support Internet access via a proxy.
For those who wish to delve more deeply, the source code may be downloaded from here and the Release Notes from here.Driving Instructions
TerrainTool is a conventional GUI-based application with a menu bar and dialog boxes to gather user-information. To get started, do the following:
- Select the required coordinate system using the Options menu. If you're not using the NASA DEM you'll also need to specify the region
- Go to Create on the File menu to specify the size, location and resolution of the mesh.
- Save the results as in Survex (.svx) or Therion (.th) format
- Use Survex to process the file and Aven to view the results in 3D. Remember to enable viewing of surface legs in Aven!
- Use the Offset command in the Options menu to fully align the terrain data with the coordinates used in your survey.
- When generating surface data in Therion format, Therion needs to be told the coordinate system used for the surface data in a form that it recognises (e.g. EPSG:27700). See the description of the surface command in the Therion Book for more details.
- Incorporate the terrain data into your survey project.
File Menu
Create... is used to calculate the terrain mesh. A dialog box is used to gather the following:-
| Item | Meaning |
|---|---|
| Grid Reference | The grid reference for the mesh, expressed in the current coordinate type. This point can be at the centre of the mesh, any of the corners or the mid-point of any of the sides (see below). An example in the correct format is shown below the text field. |
| E-W Range | The distance (in metres) between the East and West edges of the mesh. |
| N-S Range | The distance (in metres) between the North and South edges of the mesh. |
| Spacing | The distance (in metres) between adjacent cells in the mesh. |
| Grid ref is at | Specifies where current point (see "Grid Reference" above) lies in relation to the boundaries of the mesh. |
Pressing OK starts the calculation of the mesh points. If using the SRTM DEM and "Auto-download" is enabled, data files will be downloaded as needed from the USGS server. These are stored in the data directory for later re-use if necessary, avoiding the need to download the same file again. On Windows this will be in a directory called ".terraintool" in the user's top-level directory on the system drive e.g. C:\Users\mike\.terraintool and on Unix/linux at ~/.local/share/terraintool/
At the end of the calculation, results are displayed as a simple coloured relief map. The mouse position is displayed (in current coordinates) in the message bar on the bottom edge of the frame.
Lat/Long... provides a means of defining the current point in terms of latitude and longitude, rather than as a grid reference. If the point can be represented as a valid grid reference in the current coordinate system, it is used to initialise the "Grid Reference" field of the "Create..." dialog box. Likewise, the current grid reference is used to initialise the Lat and Long fields with the latitude and longitude of the current point.
Latitude and longitude values can be expressed as either
- real values in degrees (e.g. 46.25), where negative values are West / South, or
- values in degrees, minutes and seconds (e.g. 46 N 15' 22.6")
Latitude and Longitude are usually based on the WGS-84/GRS-80 datum and ellipsoid. The user may select alternatives, which will cause the lat and long values to be re-calculated.
Save as...
Once a mesh has been calculated, the "Save as..." command can be used to save the terrain data. Normally, this will be in Survex (.svx) or Therion (.th) format.
Otherwise, height values are defined for each point in the mesh. Easting and Northing values are those of the current coordinate system.
Options Menu
Coordinates...
This is used to select the type of coordinates to use. The following are currently supported:
| Coordinate System | |
|---|---|
| Austrian | The Austrian (BMN) coordinate system, in three zones |
| Irish Grid | The Irish grid system, used in both Northern Ireland and the Republic of Ireland. |
| Lambert 93 | The Lambert 93 coordinate system. A conformal conical projection occasionally used in France. |
| Lambert (5 zones) | The coordinate system most commonly used in France. Three zones (I, II and III) cover North, Central and Southern France. Zone IV is used in Corsica. A fifth zone (II-extended) covers the whole of France, at the expense of greater distortion. |
| NZMG | New Zealand Map Grid - New Zealand’s coordinate system superseded in 2010. Maps based on this are no longer available, but still widely used. Based on a conformal orthomorphic projection. |
| NZTM2000 | New Zealand Transverse Mercator, successor to NZMG. |
| OSGB | Ordnance Survey of Great Britain - the normal British grid system. |
| Slovenian Grid | A Transverse-Mercator grid used in Slovenia |
| UTM | Universal Transverse Mercator, devised by the US Department of Defense to cover the globe (except polar regions) in 60 zones. Also used by many national mapping agencies, often with a national or regional datum instead of WGS84. |
One of the design objectives of this software is to be able to add further coordinate systems with minimal difficulty.
Selecting a coordinate system from the drop-down list results in automatic selection of sensible defaults for the datum and ellipsoid. The user is free to override this selection using the other two drop-down lists. Whilst, for example, OSGB invariably uses its own datum and the "Airy Sphere", other systems are frequently used with a variety of datums. UTM, for example, is used in Spain with the European (1950) Datum and Australia with their own (MGI) datum.
Auto download enables/disables the automatic downloading of SRTM data from the USGS site. This option is disabled when the NASA DEM is selected.
The SRTM data site is organised into six regions - Africa, Australia, Eurasia, North_America, South_America and Islands (New Zealand and islands of the Pacific). As I don't have a simple method of determining the region automatically from lat/lon, you will need to manually select the right region. This option is only relevant to the SRTM DEM. It is disabled when the NASA DEM is selected.
This provides a simple way of adding a fixed 3-D offset to mesh point in the mesh as it is saved. You might want to do this because
- Your cave survey coordinates might not use the full easting and northing values
- The height values differ from "known" surface heights in your survey. SRTM heights are referenced to EGM96 (Earth Geopotential Model 1996) and may be significantly offset from a national height datum.
NASA
Creates terrain using NASADEM data. Tiles for this DEM need to be manually installed. If a required tile is missing the error message will include the name of the file that you need to find on the USGS server.
SRTM
Creates terrain using SRTM data. Any "voids" which cannot be filled by interpolation will result in gaps in the output data. If auto-download is selected and there is an Internet connection, missing tiles will be automatically downloaded from the USGS server.