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!

We were pleased to welcome Linda Ronstadt (!!!) to the HiRISE Operations Center last week. We gave her and a few of her friends & family a presentation about the HiRISE mission, and we showed them some of our images in false color, 3-D and on the HiWall. Linda was incredibly nice and enthusiastic, and she had lots of great questions for us. Turns out rock stars love HiRISE! At least, we hope she enjoyed it as much as we did!

Now you can explore Mars with version 5 of Google’s 3D exploration software (still called Earth)! HiRISE team members worked with Google to make this possible. Previously, you had to perform a few tricks to get it going, but now it is all built in smoothly. To switch to Mars. select the planet drop-down at the top center.

You can enable footprints for HiRISE, CTX, CRISM, Mars Express’ HRSC and Global Surveyor’s MOC.

By clicking on a HiRISE footprint, you can get a window with a hi-res preview and a link to the observation page on our website.

A nice addition is text from (our fellow Tucsonan) William K. Hartmann’s A Traveler’s Guide To Mars, explaining the geologic provinces on Mars (click on the green ‘hiker’ icons).

You can see screenshots and get more info from the unofficial Google Earth blog and download Windows, Mac or Linux versions from Google’s Earth site.

It looks like there is some broad-scale elevation data. Shift+up or down tilts your view, shift + right or left spins, and page up / page down zooms.

Have fun exploring Mars!

We’re in the midst of the last cycle in MRO’s Primary Science Phase (PSP). Conjunction is coming up, when Mars is behind the Sun, so we won’t be able to communicate with the spacecraft for a few weeks. We’ll get a welcome break during that time – Uplink will have two whole planning cycles off, and Downlink will get a chance to catch up with their processing.

I can’t believe it’s been two years since the last conjunction and the start of PSP! A lot has changed since we started out with those first images. (more…)

Yesterday was the 3-year anniversary of MRO’s launch. A number of people on our team sent MRO birthday wishes over email; here’s a blog card, too.

If spacecraft ages are like cat ages (and I don’t really know why they would be…), MRO is 28 in human years. That actually sounds about right to me – the mission has matured to the point where things are fairly routine (although there are always exceptions!), we’re past the difficult teenage-angst period, and we’re (hopefully!) wiser now about the way we do things, with many life lessons learned. But we’re not “over the hill” yet! In fact, we’re really in our prime right now.

In honor of this date, here’s a present – a video of the MRO launch: a smaller .ram version for Real Player (2 KB) or a larger .mov version for QuickTime (5.4 MB).

We just saw on NASA TV that the first images came down – they look great! The solar panels are deployed, and you can see a bit of the surface with some small rocks. There’s also a really cool horizon image – you can see the polygons we’ve been imaging for years, right up close! — And from a very different perspective, of course!

It seems like we’ve been preparing for the Phoenix mission for such a long time – and now it’s finally close to landing day! T-6 days according to our countdown clock! Things are getting pretty crazy here, and I thought a little overview of how the HiRISE team is supporting the Phoenix mission would be useful.

We’ve been imaging the northern plains for Phoenix since we started our mission (here’s a bunch of reconnaissance images on our website). The first images we got back showed lots of scary boulders (a close-up of one of our Transition images shown to the right), so we sampled other areas and searched for a relatively boulder-free landing spot. The area the Phoenix team finally chose is being called the “Green Valley“, not because of the “green light = safe to go” connotation, but rather because some geological maps made of the area happened to use green as the color for the valley. Perhaps coincidentally, Green Valley is also a town near Tucson, where both Phoenix and HiRISE are based. Whatever the reason, I like that the name has a lovely calm, comforting feel.

Once the Phoenix team picked out their landing site, we worked on a high-resolution mosaic of the entire 3-sigma landing ellipse (”3-sigma” means there is a 99% probability it will land within this area; see this great blog entry on landing ellipses at the Planetary Society). The Phoenix landing ellipse is shown to the left, along with the footprints of a number of HiRISE images. (This was before we were quite done with the mosaic.) These images have helped the Phoenix team characterize the regional geology and assess the safety of the landing site.

In addition to scouting landing sites, we’re also going to be involved with Phoenix during its prime mission on the ground. We’ve been planning and practicing several different ways of cooperating: (more…)

Apollo 8 commander Frank Borman visited the Lunar and Planetary Lab, including the HiRISE Operations Center, today. He’s a former Tucson resident and is giving the 2008 commencement address for the University of Arizona. The Apollo 8 crew were effectively our first interplanetary travelers (as the Earth-Moon system can be called a double planet); the first humans to travel far from the Earth and orbit another world. Their evocative pictures and descriptions of Earth as the only colorful object in the vastness of space they beheld have mesmerized people for 40 years, an anecdote that Mr. Borman recounted today. Their Christmas Eve broadcast in 1968 capped off the most tumultuous year in modern American history (elegantly reconstructed in the episode 1968 in Tom Hank’s From the Earth to the Moon).

Our P.I. gave him an overview of the Mars program, showed slides of HiRISE and also current or upcoming lunar missions. Not surprisingly, the engineering issues interested him; in particular aerobraking, heatshields, planetary protection, and the LCROSS lunar impact experiment. He mentioned how they had some doubt whether their heatshield would work (it was first capsule to come back at interplanetary speeds of around 25,000 mph). He contrasted planetary protection in the Apollo days with the great lengths we go to to remove most microbes from Mars-bound spacecraft; for the Moon landings people were more concerned about what might come back! But LCROSS will deliberately send an upper-stage into an impact trajectory; something he noted that Apollo specifically avoided by sending it on a solar trajectory.

I hope it doesn’t sound too cliché, but it was an honor to meet a real American hero! I think all of us here are real space geeks and considered it a great privilege to meet him.

(more…)

On this NPR Science Friday episode, HiRISE Principal Investigator Alfred McEwen and M.I.T. planetary geophysicist Maria Zuber discuss new results that illuminate the story of water on Mars with host Ira Flatow.

Also, available free on iTunes, are a collection of videos from the Phoenix Mission’s Open House, highlighting the University of Arizona’s Mars-related projects including UofA speakers McEwen, Phoenix P.I. Peter Smith, GRS and TEGA P.I. William Boynton, and planetary geologist Vic Baker.

Finally, during last week’s UofA football game, our marching band played a tribute to Mars and in particular Phoenix with a little ditty written by band director Jay Rees. I don’t know if a recording of the performance is available online, but here’s a snippet of the song:

“Follow the water” is NASA’s song,
UA’s happy to sing along.
We shall see what we shall see.
We might find biology!

Wednesday September 19th, we are posting our first student suggested and captioned image on the website. Last spring, students in grades 3 to 14 in schools from around the world, including Hungary, Nepal, Curaçao, India, Arizona, and New Jersey, participated in the first HIRISE Image Targeting Challenge. They suggested target locations that they thought might hold evidence of water at or near the surface of Mars in the recent past. Then the students had to analyze the returned images, submit a report as a class, and write a figure caption. It often takes a long time to get an image after it has been suggested, even for the team members, because of all of the different constraints, including the season, the roll angle limits, and because there are other instruments on MRO that we coordinate with. However, we got very lucky and were able to get twelve of the student suggested images during the spring semester. Tomorrow we are releasing the first of these images that a third grade class in Arizona suggested and analyzed. It is so exciting to me that 8 and 9 year olds are doing science, not just reading it in a textbook. Each week we will release another student image. Hopefully this has gotten some of the students excited about possibly becoming scientists when they grow up. One day some of them may become team members on a space mission.

We are starting the Fall challenge right now. If you know of a school group that might want to participate, check out the HiRISE Challenge website at: http://quest.nasa.gov/challenges/hirise/
Our first live online chat will be on September 25th.