| :: F.A.Q. | |
| What is Aster? | |
| Where can I get Aster images ? How much do they cost? | |
| What kind of data products (or correction levels) are available for Aster images ? Which ones do I need to generate a DTM with the AsterDTM module? | |
| Is it really worth while applying ortho-correction to ASTER images, at a pixel size of 15 meters? | |
| How can I import ASTER images into ENVI ? Are they already georeferenced? | |
|
|
|
|
AsterDTM |
|
| What is AsterDTM? | |
| How does AsterDTM extract elevations ? Are there any limitations? | |
| Do I need to have a stereo pair of Aster images in order be able to extract the DTM? | |
| I'm not so familiar with the concepts of ortho-correction and DTM's - how difficult is it to extract elevations with AsterDTM? | |
| What kind of products (and resolutions) can I produce with AsterDTM? | |
| How accurate are the extracted DTM's and the other products produced with AsterDTM? | |
| I need the highest precision and truly rectified images, how can I obtain these with ENVI + AsterDTM? | |
| How long does it take to extract a DTM? | |
| Can I use an external (coarse) DEM to improve the quality/speed of the DTM extraction? | |
| Can I purchase AsterDTM to run as a stand alone application? | |
| Can the AsterDTM algorithms be used with stereo pairs from other sensors (Spot5, IKONOS, QuickBird...)? | |
| What system configuration is required? Do I need ENVI to use AsterDTM? Do I need ENVI + IDL to use AsterDTM? | |
| Does AsterDTM have batch job capabilities? | |
| Is there a demo version or a trial license availabe to test AsterDTM? | |
| What is the recommended configuration? | |
| ......................................................................... | |
What is Aster? |
|
| ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) is one of the sensors operating on board of TERRA, a satellite launched in December 1999 as part of NASA's Earth Observing System (http://ASTERweb.jpl.nasa.gov/). ASTER covers a wide spectral region with 14 bands from the visible to the thermal infrared with high spatial, spectral and radiometric resolution. The spatial resolution varies with wavelength: 15 m in the visible and near infrared (VNIR - 0.55 to 0.80um ), 30 m in the short wave infrared (SWIR - 1.65 to 2.4um), and 90 m in the thermal infrared (TIR - 8.3 to 11.32um). An additional band is the key to produce Digital Elevation Models. This band (named 3B) works at the same spatial resolution of ordinary band 3 (NIR), but at a backward angle of approximately 28 degrees, producing a stereo pair for each and every ASTER image. Each ASTER scene covers an area of 60 x 60 km, and the sensor has up to 8.55 degrees of pointing capabilities. | |
|
......................................................................... |
|
Where can I get Aster images ? How much do they cost ? |
|
| Aster images can be ordered and/or downloaded at http://edcimswww.cr.usgs.gov/pub/imswelcome/ (see ASTER User Handbook v2.2 at ASTERweb.jpl.nasa.gov to learn details about ASTER data search). Currently a per image price of US$60 is charged, independent whether you download it or order it on CD/DvD/tape. File size is about 109MB (1A) to 123MB (1B), all bands included. | |
|
......................................................................... |
|
What type of data products (or correction levels) are available for Aster images ? Which one do I need to generate a DTM with the AsterDTM module ? |
|
|
ASTER standard data products available for download/purchase are
|
|
|
You will need ASTER 1A or 1B images in the default HDF (Hierarchical Data Format) data format to extract the DTM with AsterDTM. Eventually you will also see the so called expedited versions of level 1 images (1 AE, 1BE) which do not carry the band 3B and are therefore not suited for extracting DTM's. |
|
|
......................................................................... |
|
Is it really worth while applying ortho-correction to ASTER images, at a pixel size of 15 meters ? |
|
|
With an average orbit attitude of about 705 km and a swath (image base lenght) of ~60 km, pixels at the east/west border of the image are seen under an angle of ~ 8 degrees, which already accounts for a terrain displacement of about 70 meters (> 4 pixels) for an altitude variation in the order of 500 meters. Additionally, many ASTER images have been and are being acquired at a non-zero pointing angle (the instrument can look "aside"), rapidly increasing the susceptibility to terrain displacement errors (at the maximum pointing angle of 8.55 degrees, the whole image is off-nadir). Bearing these aspects in mind, it is surely meaningful to ortho-correct ASTER images. |
|
|
......................................................................... |
|
How can I import ASTER images into ENVI ? Are they already georeferenced ? |
|
|
Easy: Simply open the file with the standard Fle->Open Image File option. ENVI will automatically identify and list all file components (VNIR bands, SWIR bands etc.). For ASTER 1B images, ENVI reads the ephemeris information from the ASTER file and automatically builds the map information for the internal ENVI header. The 1B bands are orbit oriented, not map (north) oriented, but ENVI automatically takes care of the orientation angle, and you will be able to immediately navigate on the image, overlay vectors, grids etc. Please note that the map information is derived from ephemeris (space- craft) information, so this is not an image georeferenced to a certain map scale. Please also mind that you will have to convert the images to north (map) orientation if you want to mosaic several ASTER 1B images. ASTER 1A images are a kind of raw images which have to be geometrically and radiometrically corrected before they can be navigated. You can import, view and analyze 1A images just like any other images in ENVI, but there will be no geographical information related to them. The AsterDTM module applies all necessary corrections, and produces a pseudo-1B l image which is georeferenced also based on the ephemeris information. |
|
|
......................................................................... |
|
| About AsterDTM | |
What is AsterDTM ? |
|
| AsterDTM is an optional ENVI add-on module to extract digital elevation data from ASTER 1A and/or 1B images, developed by SulSoft in Research System's powerful language IDL (Interactive Data Language). Additional features are the radiometric and geometric correction of 1A images (VNIR bands only), and the ortho-correction of the orbit oriented ASTER 1B images and 1A-1B corrected bands. | |
|
......................................................................... |
|
How does AsterDTM extract the elevations ? Are there any limitations ? |
|
|
The basic priciple behind the DTM extraction with AsterDTM is the well known parallax effect - you look at an object from two different angles and thus can obtain it's third dimension. Each ASTER image contains it's own stereo pair - looking at the same terrain from two different angles, provided in the form of a 3N (nadir) and a 3B (backwards) pair of images with 15 m spatial resolution. AsterDTM converts these two bands into a pair of quasi-epipolar images which have a pixel displacement in the satellite flight direction proportional to the pixel elevation. A cross-correlation method is used to evaluate this displacement, and transform it into elevation values. This method has it's limitation where the correlation between the two images is ambiguos (typically large areas of water, sand, ice or snow) or not accessible (e.g cloud cover and associated shadows). Clouds are a major problem because they occur in both images but at different locations (time between the acquisition of nadir and backward images is approximately 6 seconds during which the cloud may have already travelled a few hundred meters). Cloud shadows also affect the correlation analysis, as there may be no cloud or shadow at a certain location in your 3N image, but in the 3B image , which destroys the correlation, yielding an area of low confiability DEM values. AsterDTM tries to fill these holes by interpolating from surrounding "good" elevation values. One can always access the quality of the elevation data by evaluating the confiability map, which shows the score (from 0 [no correlation] to 1 [best]) for the normalized cross-correlation for each pixel (i.e the "score map"). |
|
|
......................................................................... |
|
Do I need to have a stereo pair of Aster images in order be able to extract the DTM ? |
|
| No, you need only one ASTER scene!!! Each and every ASTER 1A or 1B scene carries it's own stereo pair, in the form of the two VNIR bands: 3N (image taken at nadir) and 3B (image taken at a backward angle of ~ 27 degrees) VNIR bands. | |
|
......................................................................... |
|
I'm not so familiar with the concepts of ortho-correction and DTM's - how difficult is it to extract elevations with AsterDTM ? |
|
|
It couldn't be easier :
This is it !- no control points, no external DEM needed. |
|
|
......................................................................... |
|
What kind of products (and resolutions) can I produce with AsterDTM? |
|
| AsterDTM extracts the elevation values for the user selected subset and generates an orbit-oriented, ortho-corrected digital elevation raster file at the user selected pixel resolution (15m/30m/60m), together with a confiability map (same spatial size as the DTM) so that the user can access the degree of certainty for each pixel. For ASTER 1A images the three VNIR bands are geometrically and radiometrically corrected, orbit-oriented and registered with respect to each other (pseudo 1A-1B image). In addition, the user can apply the ortho-correction to the ASTER 1B VNIR bands or the ASTER 1A->1B corrected image. | |
|
......................................................................... |
|
How accurate are the extracted DTM and the other products produced by AsterDTM ? |
|
|
The georeference datais derived only from the space craft ephemeris information. Even so, the precision we observed for hundreds of Aster images is even better than the 50 meters reported by EOS-DAC (VNIR bands), which is the x,y precision you can expect from the AsterDTM derived products. Because of no control points are used for the extraction process, the resulting DTM is a relative one, which means that one has to calibrate it to obtain absolute elevation values. However, preliminary tests indicate that the correction to be applied is less then 5% in order ot get absolute values, or in other words, the relative elevation values obtained by AsterDTM are within 5 % of the absolute values. Absolute accuracy for relative elevation values is better then +-20 m (90% confidence interval). |
|
|
......................................................................... |
|
I need the highest precision and truly rectified images. How can I derive these with ENVI + AsterDTM ? |
|
|
With the extraction of the DTM and the ortho-correction of level 1B (or 1A->1B ) images you get truly epipolar images with terrain displacements removed. You can now use ENVI's easy-to-use map registration tools to register you images by means of a scanned map, vector layers, GPS input, or manually entering coordinates obtained from paper maps. The advantage here is that you do not need to worry about the z component (elevation), like in conventional orthorectification processing, greatly reducing the number of GCP's necessary for high quality results. |
|
|
top'
......................................................................... |
|
How long does it take to extract a DTM? |
|
| Typical processing time for a full scene (1A or 1B) at default output resolution (30 m) is about 20 minutes on a PIV 2.6 GHz, 30 minutes on a Athlon XP1500 and 55 minutes on a PIII 1 GHz (all 256MB RAM or higher, Windows 2000/XP). AsterDTM shows you the estimated processing time for your system on the main control panel, and how it varies depending on the spatial subset you selected, and the output DTM pixel size. | |
|
......................................................................... |
|
Can I use an external (coarse) DEM to improve the quality/speed of the DTM extraction ? |
|
| Yes, you can import any ENVI-compatible DEM format, and load it into AsterDTM. This won't make its process much faster, but it can improve the quality of the output DTM where correlation values are low (e.g. clouds/shadows). | |
|
......................................................................... |
|
Can I purchase AsterDTM to run as a stand alone application ? |
|
| No, you need ENVI 4.3 or ENVI 4.4 installed on your computer to run it. | |
|
......................................................................... |
|
Can the AsterDTM algorithms be used with stereo pairs from other sensors (Spot5, IKONOS, Quickbird ...) ? |
|
|
The AsterDTM module itself is only aplicable to ASTER data, but the algorithms of AsterDTM terrain extraction are based on the pushbroom geometry and along-track stereo capability, technical characteristics also inherent to the sensors mentioned in your list. We plan to release modules for DTM extraction and ortho-correction for each of these sensors within the next few months. |
|
|
......................................................................... |
|
What system configuration is required? Do I need ENVI to use AsterDTM? Do I need ENVI + IDL to use AsterDTM? |
|
|
The minimum hardware configuration recommended to run AsterDTM is Pentium III 500MHz with 256 MB RAM and enough free disk space to hold your ASTER scenes, and the extracted DTM's (about 35 MB each). You need to have ENVI 4.3 or ENVI 4.4 installed, with a valid permanent or a trial license. AsterDTM has been tested on Windows 2000/XP and Linux, but it should also run on all other platforms ENVI supports. AsterDTM will run fine under ENVI RT (ENVI without IDL). You will need ENVI+IDL if you want to explore AsterDTM's batch job capabilities. |
|
|
......................................................................... |
|
Does AsterDTM have batch job capabilities ? |
|
| Yes. The highly automized concept of AsterDTM allows to extract DTM's without need for any user interaction. There are several routines very much like the ENVI routines (see ENVI Programmer's Guide) which you can use to set up a batch file (e.g. to extract DTM's from a group of ASTER images in one go), or include them in your own IDL routines. | |
|
......................................................................... |
|
Is there a demo version or a trial license availabe to test the software? |
|
|
There is a free demo version available for download which has the same look-and -feel and the same functionallity as the full version, with the following restrictions:
|
|
|
......................................................................... |
|
Recommended Configuration |
|
|
Pentium
III 1GHz.
.........................................................................
|
|