A DTM (or synonymously DEM for Digital Elevation Model) is a grid, or raster, file describing elevation values at regularly spaced points, or posts.
HiRISE DTMs are made from two images of the same area on the ground, taken from different look angles. All the stereo pairs acquired so far are available here. Not all of these have been made into DTMs due to the time-intensive process. Creating a DTM is complicated and involves sophisticated software and a lot of time, both computing time and man-hours.
As mentioned in a previous post, the great advantage of a HiRISE DTM is the high resolution of the source imagery. As a general guide, terrain can be derived at a post spacing about 4X the pixel scale of the input imagery. HiRISE images are usually 0.25 – 0.5 m/pixel, so the post spacing is 1-2 m with vertical precision in the tens of centimeters.
The three basic stages of creating a DTM are:
- Prepare the images for ingestion into the stereo software
- Triangulate the images
- Extract terrain
In order to prepare the images, we must first correct the geometry by removing any optical distortions inherent to HiRISE. Then the spacecraft pointing information at the time of each observation is gathered.
Triangulation is also called bundle adjustment. This step requires the most operator skill and time. The result is a transformation of the original images to epipolar space. What this means is that all the stereo information is now captured in the horizontal direction, or x-parallax. During triangulation, we also align the stereo model to MOLA elevations, so the end result is tied to the global elevation map produced by the MOLA instrument team. This is the same map that you see in the context map pane of every HiRISE observation page.
Once the images are triangulated, then terrain can be extracted. This step is computationally intensive, but automated, so it just takes a lot of computer time. The output of terrain extraction is reviewed for any artifacts or errors. These are edited out if possible. Since editing is extremely time-consuming, it is only done on easily corrected errors and in the areas of most interest to the researcher. The less editing we have to do, the better, so a lot of effort goes into preparing the images so that the input is as high quality as possible. The excellent contrast and value range of HiRISE imagery usually result in high quality terrain extraction that requires minimal editing.
After we have terrain, we can make other products, such as orthoimages. An orthoimage is a picture that has been orthorectified. This means that the pixels have been projected so that at each pixel it is as if you are looking directly down at the terrain. In the original stereo images, we rely on the fact that there are topographical distortions (parallax) to derive the elevations in the terrain model. In the orthoimages, all topographic distortions have been removed.
The final products are map projected using the same mapping definitions as the regular HiRISE RDR products.
A really useful (and cool) thing to do with the orthoimages is to drape them over the terrain for 3D viewing. Below is a subimage from the Newton Gullies DTM showing the imagery draped over the terrain.
You can see animated fly-throughs made with HiRISE DTMs by going to the HiClips page and clicking on the JPL Flythrough Clips. This is a great way to see and understand the geological relationships from a ground perspective.
Researchers use DTMs to take measurements and model geological processes. DTMs are very powerful research tools. In fact, almost every HiRISE DTM produced results in publication. There is a long waiting list for these products because they are so valuable and so difficult to produce. Several institutions involved with HiRISE contribute to DTM production to maximize the number of projects produced and to avoid duplication of effort.
Standard PDS products linked to the DTM project page are usually quite large files. The links provided will download the files to your system. To get a quick view of what the project looks like, click on the Extras links to see a reduced version of the products, displayed as images, grayscale, shaded relief and colorized altimetry.
Standard PDS products:
- The DTM in standard PDS image object (.IMG) format with an embedded label
- The left orthoimage at the same resolution as the DTM, in JPEG2000 format with detached label
- The left orthoimage at the resolution of the original image, in JPEG2000 format with detached label
- The right orthoimage at the same resolution as the DTM, in JPEG2000 format with detached label
- The right orthoimage at the resolution of the original image, in JPEG2000 format with detached label
Extras available in the PDS Extras directory (letters in parentheses correspond to PDS file names such as <Product_ID>.br.jpg):
- Browse (br), annotated browse (ab), and thumbnail (th) jpegs of the DTM as a grayscale image
- Browse (sb), annotated browse (sa), and thumbnail (st) jpegs of the DTM as a shaded relief image
- Browse (cb), annotated browse (ca), and thumbnail (ct) jpegs of the DTM as colorized altimetry
- Browse (br), annotated browse (ab), and thumbnail (th) jpegs of the lower resolution orthoimages
PDS product naming convention for HiRISE DTMs:
PRODUCT_ID = aabcd_xxxxxx_xxxx_yyyyyy_yyyy_Vnn
aa = DT, indicating it’s a DTM product
b = type of data
- E = areoid elevations
- 1 = orthoimage pixels from first image
- 2 = orthoimage pixels from second image
c = projection (others are possible but these are the important ones)
- E = Equirectangular
- P = Polar Stereographic
d = grid spacing (think of this as pixel scale in meters)
- A = 0.25 m
- B = 0.5 m
- C = 1.0 m
- D=2.0 m
xxxxxx_xxxx = orbit number and latitude bin from SOURCE_PRODUCT_ID
yyyyyy_yyyy = orbit number and latitude bin from SOURCE_PRODUCT_ID
V = letter indicating producing institution
- U = USGS
- A = University of Arizona
- C = CalTech
- N = NASA Ames
- J = JPL
- O = Ohio State
- Z = other
nn= 2 digit version number
Below is an example of the set of annotated browse images for the Russell Crater Dunes DTM.
The grayscale image of the DTM looks weird, if you have not looked at lots of these before, but keep in mind that the color of the pixels represents elevation. The higher the elevation, the brighter the pixel. Lower elevations are darker. The shaded relief is another way of visualizing the topography. The pixels are illuminated from a certain direction, to show the relief of the topography, rather than the elevation. It is also emphasizes any artifacts in the DTM. In the example here, many artifacts (errors) can be seen such as the faceted areas and boxes in the lower left and top of the image. These artifacts are usually caused by areas of low contrast (such as in this project) or sharply differing shadows. Most HiRISE DTMs will not have a lot of these artifacts, fortunately! The area of most interest to the researcher who requested this DTM was the long slope with the gullies, which was well-illuminated and had good contrast. So in that area, there were few, if any, artifacts. Adding color-coded elevation to the shaded relief creates the colorized altimetry map, where the lowest elevations are purple, green is the median elevation value, and white is the highest elevation. In the Russell Crater Dunes project shown here, the difference in elevation from the highest to the lowest point is almost 590 meters (~1935 ft.). That is a tall dune!!
We are happy to be able to share HiRISE DTMs with the scientific community and with the public. We will continue to release more DTMs as they become available, so stay posted!