LRO: Frequently Asked Questions About LOLA Data

If you don't see the answer to your question here, send it to geosci@wunder.wustl.edu, or check the Geosciences Node Data Users' Forum to see if it has been discussed there.

What do all those acronyms mean?

What data set do I want to use?

To get data that have already been binned in a latitude-longitude grid, use the GDRs. (Data products are in the LOLA_GDR directory; documentation is in the RDR SIS.)

To get individual altimetry tracks so you can plot them yourself, use the RDRs. (Data products are in the LOLA_RDR directory; documentation is in the RDR SIS.) The RDR data are at full resolution, unlike the binned GDR data.

If you want to do your own calibration and geolocation, start with the EDRs. Not many people want to do this. (Data products are in the LOLA_EDR directory; documentation is in the EDR SIS.)

Why are the GDR products in two different formats, IMG and JP2?

The files with names ending in IMG are in standard PDS image archive format. That means they are simple binary arrays of 16-bit signed integers, without any headers or compression or other unusual formatting. Each one is described by a PDS label in a separate file with the same name, but with the extension .LBL. (The LBL files are text files.) 

The files with names ending in JP2 are JPEG2000 files. These are provided to make it easier to use the data with popular mapping software tools. Read more about this format in JP2INFO.TXT. They also have PDS labels.

What are those files in the JP2 directories with names ending in AUX.XML?

Each JPEG2000 product is accompanied by a file named [product]PPD_AUX.XML (for cylindrical projections) or [product]MPP_AUX.XML (for polar projections). This very brief XML file contains information used by certain mapping programs, such as ArcGIS, GDAL, and Global Mapper, to display the image. It must be downloaded to your working directory and renamed to [product]PPD.JP2.AUX.XML  or  [product]MPP.JP2.AUX.XML  to allow the file to be recognized by the software.

What do I have to do to the GDR IMG data to get the elevations in meters?

As the PDS labels explain, the data values in the GDR images have been scaled. The conversion from the data value to the height in meters is

     HEIGHT = (DATA VALUE * SCALING_FACTOR) + OFFSET.

SCALING_FACTOR is 0.5. OFFSET is 1737400. These values are given in the labels.

Can't I just look at a GDR image without having to do anything special?

Yes, look for the browse version of the GDR you want. Browse images are JPEG files (not JPEG2000) that can be displayed in your web browser. There are browse versions of the RDR data too. Look in the BROWSE directory. The subdirectories underneath are in the same structure as the DATA subdirectories.

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