ASF's Radiometric Terrain Correction Project
Creation of radiometrically terrain corrected (RTC) products is a project of the Alaska Satellite Facility that makes SAR data accessible to a broader community of users. The project corrects synthetic aperture radar (SAR) geometry and radiometry, and presents the data in the GIS-friendly GeoTIFF format.
Release of new ALOS PALSAR RTC products commenced October 2014 and was completed a year later. Data included in the RTC project are Fine Beam and Polarimetric scenes in all global land areas except Antarctica, Greenland, Iceland, and northern Eurasia.
ASF has produced products at both 12.5-m and 30-m resolutions for the areas indicated in the map.
The product package includes the following contents:
- Radiometric Terrain Corrected GeoTIFF file for each polarization available
- Pixel values are gamma nought power in 32-bit floating point format
- Pixel spacing is 12.5 m for high-resolution (RT1) and 30 m for low-resolution (RT2) products
- Incidence Angle GeoTIFF, indicating the angle in radians for each pixel
- Pixel spacing is the same as the RTC GeoTIFF in the package
These two images of part of the Grand Canyon are processed from the same PALSAR data. The image on the left is uncorrected. The image on the right is terrain-corrected. In the uncorrected image, the sides of the canyon appear to be stretched on one side and compressed on the other side.
Click to enlarge. ASF DAAC 2014; Includes Material © JAXA/METI 2008.
- Layover/Shadow GeoTIFF, indicating where processing was impacted by layover or shadow
- Pixel spacing is the same as the RTC GeoTIFF in the package
- The DEM used for RTC processing in GeoTIFF format
- Browse image in GeoJPEG format
- 1000 x 1000 pixels
- Color images available when multiple polarizations are available (FBD and PLR products)
- Metadata in ISO 19115 format (xml file)
- KMZ file with 30-m overlay
Radiometric correction involves removing the misleading influence of topography on backscatter values. For example, the correction eliminates bright backscatter caused by radar reflection from steep slopes, leaving only the backscatter that reveals surface characteristics such as vegetation and soil moisture.
In the image on the right, southern slopes are initially bright, northern slopes are dark. After correction the southern slopes have a more uniform appearance. Interestingly, the northern slopes stay dark. This is attributable to different vegetation types: deciduous trees on the southern slopes, and conifers such as white and black spruce on the northern slopes.
Terrain correction is the process of correcting geometric distortions that lead to geolocation errors. The distortions are induced by side-looking (rather than straight-down looking or nadir) imaging, and compounded by rugged terrain. Terrain correction moves image pixels into the proper spatial relationship with each other. Mountains that look as if they have fallen over toward the sensor are corrected in their shape and geolocation. Terrain correction in moderate topography is shown on the right, in mountainous terrain below.
Correction of geometric distortion is sometimes called orthorectification, especially for processing of optical imagery.
Radiometric Terrain Correction
Radiometric terrain correction combines both corrections to produce a superior product for science applications.
PALSAR and Landsat 8 Fusion
PALSAR and Landsat can be used as complementary or fusion products for biomass estimation, biodiversity assessment, and forest mapping and monitoring. ASF’s RTC product provides researchers an off-the-shelf match to Landsat 8, an orthorectified product.
The fusion images below demonstrate the off-the-shelf compatability between ASF’s RTC product and Landsat 8, an orthorectified product. The first image shows the incoherent appearance of the uncorrected PALSAR L1.5 image fused with Landsat 8. The second image, a fusion of RTC and Landsat 8, shows an excellent match.
What is a Digital Elevation Model?
A digital elevation model (DEM) is the digital representation of the land surface elevation with respect to a given reference datum. DEM is frequently used to refer to any digital representation of a topographic surface and is the simplest form of digital representation of topography. DEMs are used to determine terrain attributes such as elevation, slope and aspect at any location.
Why does the DEM included with ASF’s ALOS PALSAR RTC product show different elevations than the source DEM (SRTM or NED) over the same area?
Most DEMs are geoid-based and require a correction before they can be used for terrain correction. The DEM included in an ASF RTC product was converted from the orthometric height of the source DEM to ellipsoid height using the ASF MapReady geoid_adjust tool. This tool applies a geoid correction so that the resulting DEM relates to the ellipsoid. An online tool is available that computes the height of the geoid above the WGS84 ellipsoid, and will show the amount of correction that was applied to the source DEM used in creating an RTC product.
Source DEMs Used for ALOS PALSAR RTC for Data Processing
The quality of an ALOS PALSAR RTC product is directly related to the quality of the digital elevation model (DEMs) used in the radiometric terrain correction (RTC) process. The table below summarizes the various DEM sources and the map indicates which DEM will be used in a given area.
The continental U.S., Hawaii, and parts of Alaska are covered with 1⁄3 arc-second National Elevation Dataset (NED) at a 10 m resolution. The rest of Alaska above 60 degrees northern latitude is only available at 60 m resolution with 2 arc-second NED data. The best resolution for Canada and Mexico at 30 m is given by 1 arc-second NED. For the remaining globe, the Shuttle Radar Topography Mission (SRTM) GL1 data at 1 arc-second (30 m) resolution is used. Greenland and Antarctica are mostly covered by ice and glaciers and not suitable for terrain correction. For areas in Eurasia above 60 degrees northern latitude, no suitable DEMs are available.
The DEMs were pre-processed by ASF to a consistent raster format (GeoTIFF) from the original source formats: height (*.hgt), ESRI ArcGrid (*.adf), etc. Many of the NASA-provided DEMs were provided as orthometric heights with EGM96 vertical datum. These were converted by ASF to ellipsoid heights using the ASF MapReady tool named geoid_adjust. The pixel reference varied from the center (pixel as point) to a corner (pixel as area). The GAMMA software, used to generate the terrain-corrected products, uses pixel as area and adjusts DEM coordinates as needed. Where more than one DEM is available, the best-resolution DEM is used for processing. Complete DEM coverage from a single DEM source is required for processing to proceed.