Tips for georeferencing AVHRR images in ENVI

ENVI offers a utility that georeferences AVHRR level 1b images using the geo-location data embedded in the Level 1b data file. There are a number of issues that should be considered when using ENVI’s AVHRR georeferencing utility.

First, if the AVHRR data are going to be used for any quantitative analysis, they should be calibrated before georeferencing. Furthermore, if you plan to calculate a sea surface temperature image, this must be done prior to georeferencing. Details and references for ENVI’s AVHRR calibration and sea-surface-temperature calculation utilities are available in the ENVI documentation and online help.

When georeferencing your AVHRR image in ENVI, after you have chosen the image to georeference and, if necessary, the original level 1b AVHRR image, a dialog titled "Georeference AVHRR Parameters" will appear. In this dialog window you choose the map projection for the output image, and the number of warp points.

When choosing an output map projection, it is important to carefully consider which map projection is appropriate for the location and extent of your image. Some projections are more appropriate for AVHRR scale images than others. We recommend that UTM and State Plane projections not be used for images which cover areas larger than UTM or State Plane zones. If these projections are chosen, then the resulting registration will be geographically correct, but the map coordinates in areas furthest from the origin will be well beyond normal maximum values for these projections. Also, many users choose the Geographic Lat/Lon projection, which is, strictly speaking, not a map projection at all. The pixels in an image in a Geographic Lat/Lon projection will have constant size in degree units but not necessarily in length units, because the size of a longitude degree varies with latitude. This arrangement results in extreme distortion across most AVHRR-scale images. Some map projection types that may be appropriate for AVHRR-scale images include stereographic, transverse mercator, or conic projections. Stereographic projections are commonly used for images with equal N-S and E-W extent. Transverse mercator projections are often used for images with greater N-S extent. Conic projections are often used for images with greater E-W extent.

The level 1b file contains 51 geo-location points for every line of the image. The "Number of Warp Points" referred to in the bottom of the Georeference AVHRR Parameters dialog specifies the number of embedded geo-location points that ENVI will extract and use as Ground Control Points (GCPs) in the georeferencing. The more warp points you choose to use, the more accurate the georeferencing will be, but the longer the process will take. Remember, this utility is not just ‘georeferencing’ the image it is actually ‘georectifying’ it, which is equivalent to doing a full registration warp in ENVI. The ENVI documentation recommends using 10 warp points in both the X and Y directions. If you are concerned about achieving the most accurate registration possible, however, we recommend that you choose all 51 warp points in the X direction, and perhaps one tenth the number of image lines in the Y direction. If you are georeferencing large AVHRR images (e.g., 2000 x 2000 x 5) using thousands of GCPs, be prepared to wait. The process can take hours for such large files.

After you have selected OK in the Georeference AVHRR Parameters dialog, a new dialog titled "Registration Parameters" will appear. In this dialog you select the warping and resampling methods. For large coverages, we have found that the Delaunay triangulation warp method produces significantly more accurate results. If you choose the Polynomial warp method , we recommend that you restrict the degree to 1 (or perhaps 2 in special cases), as this type of warp model will often introduce unrealistic features.

Also in the Registration Parameters dialog is a listing of Registration Output Image parameters, including the pixel size in the correct units for the chosen map projection, and the calculated output image size in map units and in pixels. We recommend that you always double check these values to make sure they make sense.

Prior to ENVI 3.6, if you chose Geographic Lat/Lon as the output projection, ENVI’s default value for the pixel size was usually incorrect. For example, when a LAC or HRPT image was read into ENVI, a pixel size of 1100 by 1100 meters was entered into the ENVI header. For the georeferencing output image, ENVI 3.5 and earlier assumed that the pixel size should remain 1100 by 1100 in the units of the chosen map projection. If a Geographic Lat/Lon projection was chosen and the output parameters were not changed, then ENVI would generate an image with a pixel size of 1100 x 1100 degrees, and the output image would contain only a few pixels. To change the pixel size to a more reasonable value, you would need to choose the Change Output Parameters button, and enter a pixel size in degrees. The correct units for the chosen map projection would be listed in the Output Map Parameters dialog under the Change Proj. button. After the pixel size was changed, the X and Y Image Size in map units that was shown in the Output Map Parameters dialog window would not be changed. Nevertheless, once you selected the OK button, the output image size in pixels would be recalculated using the new pixel size. Later versions of ENVI include automation for correct choice of pixel size, eliminating the need for you to set output parameters manually.