terraintool1

Usage

Overview

TerrainTool was written to create surface topographic data for the cave survey packages Survex (link to URL http://survex.com) and Therion (link to URL http://therion.speleo.sk) using the results of the Shuttle Radar Topography Mission (link to URL http://www2.jpl.nasa.gov/srtm/) (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) (link to URL http://www.jpl.nasa.gov/news/news.cfm?release=2009-103) 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/ (link to URL https://urs.earthdata.nasa.gov/users/new/)

With the completion of NASADEM, this release of TerrainTool uses it as its only source of data.

Having logged in to your Earthdata account, you can find the NASADEM data by going to https://lpdaac.usgs.gov/products/nasadem_hgtv001/ (link to URL 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/ (link to URL 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:-

•

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.

"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 mikemccombe <at> mikemccombe.co.uk or via the Survex list (link to URL http://survex.com/maillist.html) .

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:

1.

If you already know which tiles you will need from the NASADEM dataset, download them now and save them in the data directory. This is ~/.local/share/terraintool/ by default, or $XDG_DATA_HOME/terraintool/ if $XDG_DATA_HOME is set. Otherwise, when you try to create the mesh, TerrainTool will provide an error message to say which tile it needs and where to store it.

2.

Select the required coordinate system using the Options menu.

3.

Go to Create on the File menu to specify the size, location and resolution of the mesh.

4.

Save the results as in Survex (.svx) or Therion (.th) format

5.

Use Survex to process the file and Aven to view the results in 3D. Remember to enable viewing of surface legs in Aven!

6.

Use the Offset command in the Options menu to fully align the terrain data with the coordinates used in your survey.

7.

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.

8.

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:-

Create dialog items

Pressing OK starts the calculation of the mesh points. The NASADEM tiles need to be availale in the data directory $XDG_DATA_HOME/terraintool ("$HOME/.local/share/terraintool" by default if $XDG_DATA_HOME is not defined.

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 Systems

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.

Offset

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.