Why doesn't my ASTER scene line up properly with vector files from other software packages?

Occasionally customers ask why ASTER scenes do not seem to display or overlay properly with vectors created in other software packages. Sometimes the ASTER and vector files will display properly in ENVI, but they won't in other programs.

A georeferenced ASTER scene contains a rotation parameter. Sometimes this factor is about -9 degrees, other times it is upwards of 12 degrees. The rotation value can be seen in the Available Bands List Map Info section, when the ASTER scene is opened in ENVI. Click on the Map Info "globe" and the information will display in an expanded view. The last item listed there will be "Rotation" and its associated value.

ENVI honors this rotation factor; ENVI will read it in and will also write it out in the GeoTIFF tag information. However, ArcGIS and MapInfo do not handle ASTER rotation. Further, when a rotated ASTER scene is output as a GeoTIFF, the rotation is also accounted for in the GeoTiFF tag output. Other software programs often do not parse this tag. As a result, when you try to overlay an unrotated vector over the rotated ASTER scene, the two will not line up properly.

As a workaround to this problem of other programs not handling the rotation, you can first unrotate the ASTER scene in ENVI. To unrotate it, please follow these steps:

1. Go to the main ENVI menu and select Basic Tools->Rotate/Flip Data. Select the ASTER scene.

2. Then in the Rotation Parameters dialog box, notice that the Angle field is already filled. This value is calculated automatically and will result in a zero rotation image.

3. Output the result to a file.

4. When that image is opened in ENVI, you can see that it has no rotation. Then you can save that unrotated image as a GeoTIFF and bring the GeoTIFF into other software programs.