Earthdata Forum

Hello!

I'm very excited to use the ALOS PALSAR Radiometric Terrain Corrected (ALOS PALSAR RTC) High-resolution data for my project; however, I'm struggling to understand how to convert the downloaded files into the DEM files I need. Can someone direct me to resources to understand how to do this.

I understand that the DEM file (the one that ends in "dem.tif") included in the ALOS PALSAR RTC download was generated as part of RTC analysis and *is not* the RTC DEM final product (see references below).
- Modified RTC DEM Information and Disclaimer Section of the ALOS PALSAR RTC User Guide/Technical Information (https://asf.alaska.edu/datasets/daac/alos-palsar-radiometric-terrain-correction/)
- this post on Earth Forum: viewtopic.php?t=4094

It is my understanding that this means I need to use either one of the two radiometrically terrain corrected files from my download ("_HH.tif" or "_HV.tif") - each of which shows the RTC data for different polarizations.

Can anyone help me understand the process for converting one of these files into a DEM file? What do the values in the "_HH.tif" and "_HV.tif" files represent? Are they elevations too and if so, what are the units? I've opened one of the "_HH.tif" files in QGIS and the elevation values range from -18 to 237 with most of the pixels having a value less than 1. If there are any articles folks could help me find or videos, I would really appreciate it. I haven't had much luck in spite of numerous internet searches.

Link to data I downloaded:
https://search.asf.alaska.edu/#/?polygon=POLYGON((-79.5838%20-6.5555,-77.7784%20-6.5555,-77.7784%20-5.2972,-79.5838%20-5.2972,-79.5838%20-6.5555))&dataset=ALOS&zoom=8.284&center=-78.573,-6.555&resultsLoaded=true&granule=ALPSRP261557070-KMZ

Entry name: ALPSRP261557070
I downloaded the High res terrain corrected data.

Thank you for your help!

PS: for anyone unfamiliar with what different polarizations means (as I was ), here is an article that might be useful: https://natural-resources.canada.ca/maps-tools-and-publications/satellite-imagery-elevation-data-and-air-photos/tutorial-fundamentals-remote-sensing/microwave-remote-sensing/radar-polarimetry/9275

Hello @kestkat,
RTC products are not suitable for generating DEMs, as they do not include phase information.

The workflow for generating a DEM from SAR imagery compares the phase measurements from two acquisitions. You would need to start with two complex images (such as the ALOS PALSAR L1.1 products) acquired at different times over the same area, use SAR processing software to create an interferogram, then convert the results to a height file. NASA's Applied Remote Sensing Training Program (ARSET) has a nice presentation available that goes over the concepts of using SAR to generate DEMs and presents a workflow using ESA's SNAP software.

Unfortunately, ALOS PALSAR data is not particularly well-suited for generating a DEM. As described in the presentation linked above, the ideal SAR pair for creating a DEM would be acquired from two slightly offset places in space at as close to the same time as possible. ALOS had a repeat cycle of 46 days, and a lot of surface characteristics can change in that time, especially in areas with vegetation or strong seasonality. The most recent acquisitions were in 2011, so there may have been topographic changes since then. You must also evaluate the interferogram to determine if there are atmospheric impacts affecting the phase differencing calculations.

If you do use ALOS PALSAR to generate a DEM, make sure to check it against other DEM products (the Copernicus 30-m DEM is an excellent, freely available global resource) or benchmark datasets to make sure that it meets your requirements.