Imagery Menu
Display Image Processing Controls
PEP contains an interactive, easy to use image processing interface to manipulate any NASA data set. The image processing controls are described in a different chapter of this User's Manual.
Always Fill Map on Pan/Zoom
When you install PEP you also get over 3 GB of images from Mars. This data from NASA's Viking probes covers the entire planet at a resolution of about 200 meters per pixel. By default, whenever the map is zoomed in far enough it is automatically filled with Viking data. If you're quickly panning and zooming around looking for something you might not want the map constantly taking the time to load in Viking data you're not interested in. Click on this menu item to turn off the auto-loading of data.
Loaded Viking Images
As you pan and zoom over the map, NASA images from the Viking probes are automatically loaded off of your hard drive. NASA divides all this imagery into separate files. You probably don't care about what files have been loaded or learning more about the files. But if you do, simply slide the mouse to the right and a list of currently loaded data files will display. If you click on one of these filenames, a window will appear displaying some information about the file.
Displayed Viking Images
This menu item is just like "Loaded Viking Images" except the files listed are only those that are currently displayed on the screen.
Fill Edges On Load
Most users don't need to worry about this option. In a typical NASA Viking data file the left and right edges of the image has some blank data values. When turned on, this option causes adjacent valid data values to be copied over the blank values as the file is loaded. This results in an image without a black strip on its left and right sides.
Now for a more technical description. The vast majority of Viking data files were created using a sinusoidal projection. This creates an image that is not a rectangle, it creates a trapezoid. The edge (either top or bottom) of the trapezoid closest to the equator is longer the the side closest to the pole. The Viking PDS data files hold a rectangle. That is, the top and bottom of the image are the same number of pixels. Clearly a trapezoid image doesn't fit well in a rectangular container. NASA handles this by adding "null value" pixels on the left and right sides of the trapezoid to turn it into a rectangle of data. Since the null values are zeros, they show up as black pixels. If you want to see the original NASA data, just turn this option off and PEP won't change the null values as files are read in. Naturally, these black stripes are a bigger problem at higher latitudes where the difference between the length of the top and the bottom of the trapezoid is larger.
Download Hi-Res THEMIS Images
So far, the images we have discussed are from the Viking probes. This is a great data set gathered in the 1970s that provides global coverage of Mars. Its limitation is its resolution. Each image pixel is around 200 meters in on a side. So an object has be be bigger than two football fields to show up as a tiny dot. If you're looking at large features, this isn't a problem.
If you need to see more detail then you need something with higher resolution then Viking images. Fortunately NASA has already built the THEMIS high resolution camera, put on a probe named Odyssey, put Odyssey in orbit around Mars and created an on-line archive of high resolution THEMIS images.
To get high-res images for an region first pan and zoom the map to whatever area you are interested in. Then, select the "Download Hi-Res THEMIS Images". PEP will send a query over the internet to the THEMIS archive located at Arizona State University. This query contains the current area displayed on the map, defining your area of interest. The query's response contains a list of files. PEP then begins downloading each file. As each file download is complete, it is displayed on the map. All the hi-res THEMIS images are placed in the Hi-Res Imagery layer.
If the server has more then 25 THEMIS image file for your area of interest a warning is displayed asking if you really want to download all these files. Generally when you get this message you should click "Cancel", then zoom in on the map to make your area of interest smaller and select "Download Hi-Res THEMIS Images" again.
By default THEMIS images are displayed in the Hi-Res Imagery layer. Like all layers, this layer is initialized with several image processing filters. By default, these filters convert the THEMIS data to transparent yellow images. This is done so the THEMIS images aren't lost amongst the background of Viking images. Generally, after you download some THEMIS images you will want to change the Hi-Res Imagery layer's False Color filter to be gray and opaque.
THEMIS images don't perfectly correlate with Viking images. This means, for example, that a crater rim on the THEMIS image won't match up with the rest of the crater rim on a Viking image. THEMIS images can have horizontal black lines in them, an "echo" where part of an image is incorrectly repeated or rarely unintelligible garbage. Still, many THEMIS images can be stunningly beautiful.
What about MOC? MOC is a high resolution camera on NASA's Mars Global Surveyor probe. As far as I know, these images are not yet in an on-line archive.
Save THEMIS Images Locally
Being able to download and display THEMIS images is great. However, the download can take a while and sometimes you might not be on-line. So, PEP includes the ability to save downloaded THEMIS files on your hard drive. Just click on "Save THEMIS Images Locally" and you will be prompted for a name. All the recently downloaded THEMIS images saved in a directory using the name you provided. The next time you want to see these images, simply reload the files.
Load Local THEMIS Images
With this menu option you can load in THEMIS images that you previously saved. When you slide the mouse to the right of this menu item you'll see a list of all the image groups you have saved. Click on which ever one you want to load.
Currently Loaded THEMIS Files and Currently Displayed THEMIS Files
These options are a copy of the Viking menu options described above except they with work with the THEMIS data. If you want to see what THEMIS files are loaded, find out which file corresponds to which tile on the map or look at a PDS header you can use these menu items. This is particularly useful if you care about which THEMIS images are infra-red and which are from the visual region of the spectrum.
The ability to unload THEMIS images is also useful. Often the downloaded THEMIS images will overlap and some will be at a higher resolution then others. By using the highlight option you can identify any unneeded files and unload them before you save the rest of the THEMIS images locally.