Working with the Data
Geolocating the Data
There are two stages of geolocation possible.
Rough geolocation information is provided in the rad_geom files. These files are produced on orbit and are immediately available with the data files. At this point the data are geolocated to within about 10 kilometers. Work is in progress to improve the accuracy of the geolocation in these files.
The best geolocation can be obtained by first geolocating with the rad_geom file and then shifting (translating and rotating) the resulting scene to remove any remaining offset. This is done by comparing landmark locations in the HICO scene with those in a reference image. This geolocation step is scene-specific and manual.
The following instructions describe the geolocation procedure using ENVITM software1 (version 4.2). Similar processes are likely available using other remote sensing software.
Geolocating with rad_geom in ENVI software
- Launch the ENVI software.
- Load the data file to be geolocated using File > Open Image File from the main menu.
- Load the rad_geom file using File > Open Image File.
- From the main menu, select Map > Georeference from Input Geometry > Build GLT. For the X geometry band, select longitude from the loaded rad_geom file, and for the Y geometry band select latitude.
- In the Geometry Projection Information window that appears, select Geographic Lat/Lon from the input list, and Geographic Lat/Lon from the output list. For both lists, the Datum and Units can be left at WGS-84 and Degrees respectively.
- In the Build Geometry Lookup File Parameters window that appears, leave the default values for pixel size and rotation. The output can be kept in ENVI memory or sent to a file. Select OK.
- Wait several minutes ..... new entries will appear in the Available Bands List.
- From the main menu, select Map > Georeference from Input Geometry > Georeference from GLT.
- Select the geometry file that was just created. In the File Information box, the Description should say Georeferencing Lookup. Select OK.
- In the Input Data File window that appears, select the data file to be geolocated (e.g. the *.hico.bil file). Select OK.
- In the window that appears, leave the defaults and choose whether to save the output to memory or a file. If working with the large *.hico.bil file, it is recommended that the output be sent to a file rather than memory.
- Wait about a minute ..... new entries will appear in the Available Bands List that begin with Georef.
- The file has now been geolocated to within a a few kilometers. For more accurate geolocation, continue on to the next section.
Geolocating by Comparing with a Reference Image in ENVI software
The process described below requires user interaction.
After geolocating the image following the steps above, an offset remains. This offset can be removed with a translation/rotation by co-locating features in the HICO scene with the same features in an accurately geolocated image from another source. In the steps below we use version 1.6 of the Google EarthTM mapping service (freeware) as a source for the reference images1. Alternative software or images could be used instead.
This geolocation step is only possible using scenes with obvious features (ground control points) at similar elevations, preferably near sea level. Do not use mountains as ground control points. The best scenes to use are coastal scenes with identifiable landmarks and man-made coastal structures such as bridges and wharves. Ocean scenes or cloud-covered scenes cannot be geolocated in this manner. Land scenes can be geolocated if the scene is relatively flat. At least three features are required. Note that Google Earth images can be several years old, so be careful when comparing features that may have changed over time, particularly man-made ones.
Locations with significant topographical features will not be properly corrected. For such areas one needs to use a digital elevation model (DEM) or digital surface model (DSM) in the orthorectification process. Orthorectification is not described here; please consult the help section of your image processing software for more information.
- In the ENVI program, load a HICO scene that has been geolocated with the LonLatViewAngles file (as described in the section above). The LonLatViewAngles geolocation step is required since it corrects for numerous effects such as view angles. The subsequent steps below only perform a simple translation/rotation.
- From the Available Bands List, display one of the geolocated HICO bands (or an RGB image) by selecting the band(s) of interested and clicking on Load Band.
- From the main menu, select Map > Registration > Select GCPs : Image to Map. Select Geographic Lat/Lon as the projection, with datum WGS-84 and units of Degrees. Set both X and Y pixel sizes to 0.0009 degrees (approx 100 m) and click OK.
- Navigate within the image display window to find a suitable feature and center it in the zoom window. Click on the feature in the zoom window to record its sample/line coordinates in the Ground Control Points Selection window. Do not close the ENVI interface.
- In the Google Earth mapping service, navigate to the same region. If the lat/lon location of your cursor doesn't show up at the bottom of the window, check your settings under Tools > Options > 3D View. Make sure the lat/lon values are displayed in decimal degrees. Place your cursor over the feature and record the lat/lon values.
- Copy the lat/lon values recorded from the Google Earth window into the appropriate text boxes in the ENVI Ground Control Points Selection window. Use negative values for west or south coordinates. Click on the Add Point button.
- Click on the Show List button to see the list of points added.
- Repeat steps 4 - 6 for as many features (ground control points) as possible throughout the scene. There are errors inherent in finding the exact location of a feature in both the HICO scene and the reference scene. Generally, selecting more features will help to decrease the average error. After adding at least three ground control points, click on the Predict button in the Ground Control Points Selection window to show the new (predicted) coordinates for those points and their associated RMS errors. Remove any points with high errors.
- From the Ground Control Points Selection menu, select Options > Warp File .... Select the HICO file to be translated/rotated. This should be the file from step 1 that was geolocated with the LonLatViewAngles file.
- In the Warp Parameters section of the Registration Parameters window, set the Method to RST (Rotation, Scaling and Translation), choose an output filename and click OK.
- Wait a few minutes .... new bands will appear in the Available Bands List window beginning with Warp.
- The file has now been geolocated as accurately as currently possible.
For more information, see the ENVI tutorial: "Image Georeferencing and Registration".
1 Software products listed on this webpage are not meant to imply endorsement by the HICO team or the Office of Naval Research. The products named are provided only as examples of software that may be used.