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:

1. Prepare the images for ingestion into the stereo software
2. Triangulate the images
3. 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
where
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[1]
yyyyyy_yyyy = orbit number and latitude bin from SOURCE_PRODUCT_ID[2]
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!

And now, for all of those curious minds out there, a brief overview of the HiGlyph Pipeline:

• Anaglyphs are created in a three-step process. The first step is to take the two images of the stereo pair and map project them. This helps the pipeline determine which image will be the left image and which will be the right image in the anaglyph.

• The second step takes the two images and looks to see if there are any improvements that can be done on them before putting them together. If there are not, the images move on. Often, due to the difference in viewing angle, the two images do not have a 100% overlap. Thus, to make the image a bit neater and easier to see, we trim off the excess portion of the image (the parts that do not overlap) and then assemble them so the left image is the red and the right image is the blue/green.

• The third and final step of this image processing is simply to prepare the images you see here and to update our catalog.

Seems complicated, right? Well luckily we have wonderful programmers that create these intricate programs. All I have to do is create a list of these images and run them through this pipeline. What really makes my job interesting is the validation process!

I have had the pleasure of being able to look at all 362 of the anaglyphs we have released today. But, you might ask, aside from looking super cool in 3-D glasses , what does it take to validate these anaglyphs? Well, at the beginning of this process the student validators and I got to ask that very same question. Since HiRISE has never had software to create images like this before, we played lab rat and came up with an entirely new technique for validation.

• You may notice that when not wearing the 3-D glasses there is a bit of a horizontal shift in the anaglyph. This shift is good because it is what allows us to see the image in 3-D. But, since the map projection of this process is not always spot on, we sometimes wind up with a vertical shift too. This is bad! Since most of us do not have googly eyes, this makes the image very difficult to see. With our validation process, we have to spot this out and fix it so that you do not have to strain your eyes (well… not too much at least ) in order to see the anaglyph.

Well, with that said, I leave you to your regularly scheduled HiRISE browsing! Enjoy!

Most of the presentations from the workshop have also been posted here. It’s rare to see one without some HiRISE images!

Here at HiRISE, we are interested in this for several reasons – not only are some of our team members involved in the site selection, but HiRISE data have been integral to the process. HiRISE images have been used to study the small-scale geology of the sites, which is very powerful when combined with CRISM and other data sets to determine composition and mineralogy. HiRISE data has also provided calculations of the slopes and rock abundances around the landing sites, both of which are critical for the safety of the lander. We’ve been doing reconnaissance (the R in MRO!) of all of these sites since we started our primary mission! Here are some of the data we’ve produced for the MSL project.

• Lots of images! (links to a search for ‘MSL’ in our catalog; you can also search for the individual site names)
• Anaglyphs (red-blue 3-d of stereo pairs – these are so fun! )
• Digital Elevation Models (DEMs), which are painstakingly built from our stereo paired images. (This site doesn’t have any MSL DEMs posted yet, but they should be coming soon!)

I know people here have their favorites among these seven sites – what’s yours?

On the main HiRISE site, we’ve posted a list of all the stereo pairs we’ve acquired & released. There are 467 of them! Now you can look up whether your favorite image is part of a stereo pair or not. (I don’t think the list is searchable yet, but it’s sorted by the observation ID of the first half.) There is also a PDF document on the page (click here to download directly) with more information about our stereo images. It includes instructions for using these stereo pairs to make your own red-blue anaglyphs (the ones you view with the 1950s 3-D glasses). To the right is one of those anaglyphs. This is part of a delta in Eberswalde Crater from the image PSP_001534_1560, linked on that page. (Search our catalog for ‘anaglyph’ to see even more.)

Wondering how we get stereo imaging?

It’s the same principle as seeing 3-D with your eyes: we need parallax to get the third dimension. To do this with only one “eye” (the HiRISE camera), we observe the same spot on Mars twice, from different directions. Usually the first half of the stereo pair is in a small off-nadir roll in one direction, and then the second half is taken on a larger roll in the other direction.

We can’t take two images at once, so we have to wait for the spot to be viewable twice – that can sometimes happen in the next two-week cycle, but usually it takes several cycles. Sometimes conflicts with other instruments or spacecraft activities prevent us from completing the stereo in enough time, and we have to “abandon” the first half. We’ve started calling those “left at the altar.” It’s very sad.

There’s a time limit on completing stereo because over time, the shadows change with the Sun’s seasonal movement. If the pattern of bright and dark material on the ground changes for any reason, the two halves of the stereo become impossible to correlate. For example, if a dust storm or a dust devil moves dust around, or if frost appears or disappears, the stereo is ruined. So once we take the first half of a stereo pair, the clock is ticking on completing it.

It would be wonderful if we could make every image a stereo observation, but we have to save these for our very highest-priority scientific goals. HiRISE will image less than 1% of the surface of Mars in the primary mission – so every image is precious! When we do stereo, we’re repeating coverage over the same ground. These stereo images have a very high scientific value to the team, but sometimes it’s still hard to think about trading that for an observation of another spot on Mars that has never before been imaged at high resolution. We have to make some tough decisions!

It was another special sequence that had to be specially designed, commanded, tested and re-tested before it executes on board the spacecraft. Many people from JPL and LMA worked to make this happen, as well as almost everyone on the uplink team here at HiROC. In addition, the date chosen meant that people had to come in and work on a holiday to support it. So we were thrilled when the images arrived Sunday night, and we saw that they are PERFECT! The focus, timing, and pointing were bang-on, and we got a beautiful exposure of the satellite in both images.

I wish I could give you a sneak peak, but we’re still processing the images. The downlink group has to do a lot of work on special images like these. Because they’re not normal Mars images, the normal calibration routines and processing pipelines can’t be used. Much of this work has to be done by hand. We’re also trying to get some additional products together that we don’t usually release; I think you’ll like them!

So this is really just a teaser. We’re planning on releasing the Phobos images next week, so keep an eye on the website! In the meantime, here’s a warmup act: views of Phobos from previous missions, MGS (above right) and Mars Express (left).

ETA: The data are now available here: http://www.uahirise.org/phobos.php