Visualisation of Multi-Resolution Public-Domain DEM Data Using Surfer
Lester Anderson B.Sc. M.Sc. D.I.C
Structural geologist and cartographer
As a professional geologist, I have spent many years making maps for all types of data from elevation models to geophysical data (e.g. gravity and magnetics), and all too often it is apparent that we all have different viewpoints on how best to display data in map form.
The transition from hand-drawn maps to computer-based mapping has made a big difference in both speed and presentation of data. However, it does lead to issues with the fact that data comes in a variety of spatial resolutions, and particularly so for digital elevation model (DEM) data. As an example, one can often see full-resolution SRTM data (~90m) plotted with that pixel scale for regional maps, where such detail is not necessary, but is done as a matter of convenience in many cases. Computing power now is at a level that handling very large raster files is not an issue, but deciding on the best resolution of data for the GIS project or map print out, is often overlooked.
This article highlights the use of a web-based, and Java application, that allows the user to refine the spatial scale of the data to the study at hand, and ultimately the final scale of your map. Ryan et al. (2009) devised a more systematic approach to topography (and bathymetric) data, to allow the pixel resolution to be a function of the area of interest; they derived synthesis of multi-resolution topography data termed ‚ÄúGlobal Multi-Resolution Topography‚ÄĚ (GMRT).
Global Multi-Resolution Topography (GMRT) data
The GMRT data is a global DEM model that includes a vast range of data including high-resolution bathymetry, with topographic elevations at various resolutions depending on location. The data is accessed via a web-portal (http://www.marine-geo.org/portals/gmrt/) from which the user can generate a map with a simple selector tool on a map and then have the option to save this as a grid in a variety of formats, with compatibility with the Surfer program from Golden Software (http://www.goldensoftware.com). GMRT give all users free access to data that would otherwise be hard to collate, and can be utilised in various software packages; the Surfer 2D and 3D mapping software is a great tool to explore such data.
The following image shows the general search tool for the web application, as one zooms closer in on the data the spatial (pixel) resolution changes. Using this example over the Azores region, a large view may have a pixel resolution of 3km, but view a smaller area and this is increased to 250m:
|Download global DEM models from the Marine Geo web portal.|
The various grid data formats include GMT, GeoTIFF and ascii Raster, in addition to being able to define the grid resolutions available for the area of interest. At this point the user should consider what is the best resolution for the map being generated. New data are constantly being added to GMRT, so it is a constantly evolving data source.
For more datasets, including LiDAR data for some regions, the GeoMapApp Java application can be used (http://www.geomapapp.org/). In this case an application is run to access the data, including DEM data, geophysical (gravity, magnetics, tomography) and lots more. Very high-resolution LiDAR data can be accessed for some areas up to 1m resolution!
The user interface, displayed below, of the GeoMapApp is easy to follow and again allows the map view to be saved as a grid; details of the procedures are on the application website.
|Access additional datasets, including LiDAR data for some regions, with GeoMapApp.|
Integrating Multi-Resolution Data in Surfer
Having decided on your map region and saved the data in the format of choice, this can then be readily imported into Surfer. At this stage one has the option to treat the data as a normal raster map, to overlay other data or apply further processing (e.g. morphometric analysis).
By way of comparison, the image of Island of Hawaii is derived from the Sandwell and Smith topography (version 18) (Smith and Sandwell, 1997) as an inset view. It is clear that the broad elements of the topography and bathymetry are visible, the nominal resolution of 1 arc minute (~1.85 km) is not useful at a fine scale.
The GMRT data displayed as a Surfer 3D surface is vastly more detailed, such that small-scale features associated with the southern rift zone and the "Big Crack" can be defined. As with all data in Surfer, the choice of colour-scale should be appropriate for the data. In this example, I used a custom colour ramp of my own design that was originally developed for GMT but adapted for Surfer such that we have a clear definition of topography and bathymetry and the zero elevation.
Depending on the analysis being undertaken, the choice of illumination vector may need to be adjusted to highlight the key features of interest. The Surfer application has a vast array of tools, more so in the latest release (version 15), but even utilising an earlier version (e.g. version 10) you can still generate beautiful maps and surface views.
|Island of Hawaii GMRT data displayed as a Surfer 3D surface.|
The Azores region is a good example of the GMRT data where we have a variety of data sources that included multibeam bathymetry over the Mid-Atlantic Ridge and the detailed survey data around the islands. In Surfer, it is easy to add more cartographic details such as place names from a separate file. The level of detail increases as the view gets closer; a function of the multi-resolution database. From a cartographic perspective the GMRT system negates the need to always have the highest resolution data for regional views, and the associated large files; always select the resolution appropriate for the area of interest.
|The Azores region has a variety of data sources including multibeam bathymetry.|
Surfer allows the user to combine various data to aid in regional interpretations, in the following case a Surfer base map was created from the California fault database, and imported as a shapefile. The major Californian fault systems are well-defined from the GMRT data at the larger scale. Here again, it is important to choose the best view angle to highlight the features or lineament trends in the data. Generally the data for the United States is at about 30m, with additional detail from ASTER data (10m).
Using Surfer, it is very easy to compile complex projects in much the same way as one would do with GIS programs. In the case of California, we can use the 3D surface view or a shaded relief map to map the major faults and show the correlation with the landscape morphology.
|3D surface view of the Californian fault system.|
A regional view of the Grand Canyon using a shaded relief map in Surfer, with an image overlay and transparency applied. Combining Surfer and GMRT data is a perfect combination, and if your map needs to be larger or a more detailed scale, it is simply a case of going back to the web-portal (or application) and adjust the details of the area of interest.
|Regional view of the Grand Canyon.|
GMRT data via the GeoMapApp allow access to high-resolution LiDAR data for selected regions; more areas are always being added. The same grid resolution choices are available depending on the area of interest, from large to small. The following example is based on the LiDAR data for the Crater Lake area of Oregon, the data has a full resolution of 3m for areas within the Cascades.
Here we can see how the spatial resolution varies dependent on the area selected. So at a larger view the data from GMRT will be at 50m, a closer view pushes this to 10m (red outline), and finally a detailed view of Wizard Island at 3m.
|LiDAR data for Crater Lake area of Oregon.|
In conclusion, I would certainly recommend that users get to working with GMRT data for rapid map generation. It is the perfect choice for Surfer users, no matter what level of skill. Given that the data are always going to be revised as new surveys are added, you can be sure to have the latest information.
Whether you are a geologist or pure cartographer, there is much to discover about our planet; have fun mapping!
Ryan, W.B.F., S.M. Carbotte, J.O. Coplan, S. O'Hara, A. Melkonian, R. Arko, R.A. Weissel, V. Ferrini, A. Goodwillie, F. Nitsche, J. Bonczkowski, and R. Zemsky (2009). Global Multi-Resolution Topography synthesis, Geochem. Geophys. Geosyst., 10, Q03014, doi: 10.1029/2008GC002332
Smith, W. H. F., and D. T. Sandwell. (1997) Global seafloor topography from satellite altimetry and ship depth soundings, Science, v. 277, p. 1957-1962.
Sandwell, D. T., R. D. M√ľller, W. H. F. Smith, E. Garcia and R. Francis (2014). New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure. Science, Vol. 346, no. 6205, pp. 65-67, doi: 10.1126/science.1258213.