From LP DAAC: Change in the ASTER DEM Production Software at the LP DAAC

Background

Since April 2001, the LP DAAC has offered a digital elevation model (DEM) produced from ASTER Level-1A data as a standard data product. Two versions of the product were available: a relative DEM, production of which required no ground control points (GCPs), and an absolute DEM, which required user-supplied GCPs. Unlike other ASTER standard data product generation systems, the DEM generation system was designed to produce only one or two DEMs per day. The ASTER DEMs were the only higher-level products that were archived and available for ordering by all users.

The ASTER relative DEM turned out to be one of the most popular ASTER data products among users, and the backlog of orders for that product grew very large. In addition, the LP DAAC became aware of an apparent degradation in the accuracy of its ASTER relative DEM over time.

Decision to Change the ASTER DEM Production Software

In late 2005, the decision was made to change the ASTER DEM production software at the LP DAAC. That decision followed an extensive assessment of the accuracy of ASTER relative DEMs produced by a variety of ASTER DEM generation software, and was made in response to a desire by NASA management to increase the rate of ASTER DEM production at the LP DAAC. The new software implemented and released on May 24, 2006 has the capability to produce 50 or more ASTER relative DEMs per day in batch mode operation. The accuracy of the new DEMs meets or exceeds accuracy specifications set for the ASTER relative DEMs by the applicable Algorithm Theoretical Basis Document (ATBD). Validation testing has shown that DEMs produced by the new system frequently are more accurate than 25 meters RMSExyz.

Important Characteristics of the LP DAAC ASTER DEM Product

In addition to higher production throughput and improved accuracy of the relative DEM, users need to be aware of other important characteristics of ASTER DEM generation at the LP DAAC:

No Absolute DEM. LP DAAC no longer produces an ASTER absolute DEM (i.e., those produced with user-supplied GCPs). Demand for the previously available ASTER absolute DEM was very low, and product accuracies frequently were diminished by poor quality user-supplied GCPs. Users likely will find that the DEMs produced by the new DAAC system have accuracies approaching those specified in the ATBD for absolute DEMs.

Product Differences. Compared to ASTER DEMs previously available from the LP DAAC, users likely will note some differences in ASTER DEMs produced by the new system, because DEMs now are produced automatically, with no manual editing. Larger water bodies are detected and typically have a single value, but they no longer are manually edited. Any failed areas, while infrequent, remain as they occur. Cloudy areas typically appear as bright regions, rather than as manually edited dark areas. For best results and to the extent possible, users are reminded to select high-quality, cloud-free Level-1A data as input for generating DEMs they order.

ASTER DEM Archive Eliminated. The previously existing ASTER DEM archive is no longer available. The ASTER DEM available from the LP DAAC is strictly an on-demand product.

Geodetic Reference. ASTER DEMs produced by the LP DAAC are referenced to the Earth's geoid using the EGM96 geopotential model.

Product Format. ASTER DEMs are available from the LP DAAC in GeoTif format only.

Product Ordering. ASTER DEMs are available from the LP DAAC through the EOS Data Gateway (EDG) as an on-demand product only.

Distribution Media. As an on-demand product, the ASTER DEM is available to users via FTP-Pull only.

Product Cost. The cost of an ASTER DEM remains $80 per product.

Download Procedure

The general EOSDG download procedure can be found at the WIST Ordering Tutorial

Step 1: Visit NASA WIST and open an account according to the directions on the page.

Step 2: Enter as a registered user or as a guest.

Step 3: Select radio button: ASTER.

Step 4: Select Data Set: ASTER L1A RECONSTRUCTED UNPROCESSED INSTRUMENT DATA V003

Step 5: Select the latitude and longitude bounding coordinates of your search area.

Step 7: Set maximum granules returned = (your choice). I usually start with 5. (See comment below.)

Step 8: Allow searches to run for (your choice) minutes. Try 5 minutes to start.

Step 9: Start the search.

Step 10: If data granules are returned you must examine them. If none are returned, try increasing the search time. Just because your query did not produce results does not necessarily mean that the data is not there. It may simply mean that the search timed out before it located your data. Don't give up until your search has run at least an hour, if the search utility will let you

Step 11: Look at the latitude and longitude of the returned granules to see how closely they match your target area. Then look at the percent cloud cover. The higher the number, the greater the cloud cover. Anything greater than about 10% may be of questionable value for DEM production. If you want to you can look at the browse image from which the DEM was produced by clicking the "View Image" button. This will also give you some indication of how useful the DEM will be.

Step 12: When you have decided what you want to order, check the 'Select' box next to your selections and press the "Add to Cart" button. Then press the 'Order Options' button. From the next screen select "Option 10" AST14DEM (Relative DEM) (ASTER On-Demand L3 Digital Elevation Model, GeoTIF Format) V003. Media Types : FtpPull, DLT, DVD

Step 13: Check the desired media type. In most cases it is most expedient to check "FtpPull". This will allow you to download your order from an anonymous ftp site. Then click "OK, accept my choice and return to shopping cart". When the next screen comes up, click 'Go to Step 2: Order Form'.

Step 14: Proceed to Step 2 and fill out your order information. Review the data on the Order form and then click 'Go to Step 3: Review Order Summary. Make sure the information is correct on the Order Summary form, then click 'Go to Step 4: Submit Order'.

Step 15: Order your data. If placed successfully, you will see an 'Order Submitted' form. Your order will be processed and you will receive an acknowledgment of the order by email. When the order is filled, you will receive a second email with the address of an anonymous ftp site that will have your ASTER DEM in GEOTIFF format.

I have shown a series of order screens to the right. If you are not seeing something a lot like these during the process, you are doing something wrong.

Step 16: When you have been informed that your order processed successfully and you have been directed to the ftp site, login to the site using ws_ftp or your browser. (It is no longer possible to access the site using the ftp prompt in the DOS window.) The following directions use the free utility ws_ftp, which I have found is reliable if the following directions are followed. I have not been successful in using my browser to access ASTER data, but that is probably because I do not have it set up for passive transfer mode (see below).