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.
Product Documentation
Extent and concentration of processed RTC scenes. PALSAR has two fine beam modes, single polarization (FBS) and dual polarization (FBD), as well as quad polarization, also known as polarimetric mode (PLR).
RTC Products
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
- Best resolution SRTM or NED source DEM available, with geoid correction applied
- Pixel spacing is the same as the RTC GeoTIFF in the package
- Technical information is available in the RTC Product Guide and the ATBD (algorithm) information
- 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
Note: More information on the DEMs used for RTC processing can be found in the PALSAR RTC DEM Information section.
Image Corrections
Radiometric Correction
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
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.
PALSAR RTC DEM Information
The Digital Elevation Model (DEM) file included in the ALOS PALSAR Terrain Corrected product package is not generated from the PALSAR data itself. It is a copy of an existing DEM that was modified and then used for the radiometric terrain correction process. The pixel spacing of the source DEM was adjusted to match that of the Terrain Corrected image it is packaged with and is not an indication of the resolution of the DEM.
The source DEMs used for the ALOS PALSAR Terrain Corrected products are from either the Shuttle Radar Topography Mission (SRTM), or the National Elevation Dataset (NED). Refer to Table 1 (below), “DEMs used for ALOS PALSAR RTC processing,” to determine which DEM dataset is used for your RTC products and the native resolution of the source DEM.
The DEM tiles covering the extent of the Terrain Corrected scene are extracted from the source DEM and mosaicked into a single DEM raster. This mosaicked DEM is processed to apply a geoid correction (see Fig. 1), then resampled to match the output pixel spacing of the Terrain Corrected product.
The DEM files packaged with the Terrain Corrected products are only intended to be used as a reference for interpreting the results of the radiometric terrain correction process. They should not be used in place of a regular DEM, as the elevation values are altered by the geoid correction in preparation for RTC processing.
Figure 1. Most DEMs are geoid-based and require a conversion before they can be used for terrain correction. The DEM included with 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.
The GeoidEval Utility is a free online tool that computes the height of the geoid above the WGS84 ellipsoid at a specific location and will show the amount of correction that was applied to the source DEM used in creating an RTC product.
About the source DEMs used for ALOS PALSAR RTC processing
The quality of an ALOS PALSAR RTC product is directly related to the quality of the digital elevation model (DEM) used in the radiometric terrain correction (RTC) process. The table below summarizes the various DEM sources and the map indicates which DEM was used in a given location.
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 was only available at 60 m resolution with 2 arc-second NED data. The best resolution for Canada and Mexico at 30 m was provided by 1 arc-second NED. For the remaining globe, the Shuttle Radar Topography Mission (SRTM) GL1 data at 1 arc-second (30 m) resolution was used. Greenland and Antarctica were mostly covered by ice and glaciers and not suitable for terrain correction. For areas in Eurasia above 60 degrees northern latitude, no suitable DEMs were available.
Table 1. DEMs used for ALOS PALSAR RTC processing
| DEM | Datum | Coverage Area | DEM Resolution | Product Resolution | Resampling Approach |
|---|---|---|---|---|---|
| NED13 | NAVD88 | CONUS, Hawaii, parts of Alaska | ~10m (1/3 arc sec) | 12.5 m | No resampling |
| 30 m | Down-sampled DEM to 30-m pixel spacing | ||||
| SRTMGL1 | EGM96 | Latitudes between 60 N and 57 S degrees | ~30 m (1 arc sec) | 12.5 m | Up-sampled 30-m mapping function to 12.5-m mapping function |
| 30 m | No resampling | ||||
| SRTMUS1 | EGM96 | CONUS, Hawaii, parts of Alaska | ~30 m (1 arc sec) | 12.5 m | Up-sampled 30-m mapping function to 12.5-m mapping function |
| 30 m | No resampling | ||||
| NED1 | NAVD88 | CONUS, Hawaii, parts of Alaska, Canada, Mexico | ~30 m (1 arc sec) | 12.5 m | Up-sampled 30-m mapping function to 12.5-m mapping function |
| 30 m | No resampling | ||||
| NED2 | NAVD88 | Alaska | ~60 m (2 arc sec) | 12.5 m | Up-sampled 30-m mapping function to 12.5-m mapping function |
| 30 m | Up-sampled DEM to 30-m pixel spacing |
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 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 was available, the best-resolution DEM was used for processing. Complete DEM coverage from a single DEM source was required for processing to proceed.
