Version 6 (modified by benj, 15 years ago) (diff)

Added processing speed/file size answer

Frequently Asked Questions

This page is designed to complement the Help page. If you ask us one of the questions on this page, you're likely to get an answer copy/pasted from here. If you have read this answer and are still unclear, please mention that you've looked at this page when you contact us so that we don't just give you the same answer again.

Q: When I geocorrect data, Azgcorr stops and complains that the flightline has a turn in it - how can I process my data?

This means that the aircraft turned without turning off the sensor. Azgcorr was designed to process reasonably straight single survey lines and is not intended for use on flights including large turns - attempting to process such lines causes problems. There are two solutions to this:

  1. (Easy solution) Process the flightline with an extra "-bend" option to azgcorr. This will override the turn check and allow processing to proceed. Be aware that this may cause the processing to segfault, and if it doesn't then it may not produce correct positioning. You are advised to check your results against vectors or other known good data.
  1. (Harder solution) Process the flightline in pieces and leave out the bit that's got the bend in it. The easiest way to do this is to process as in 1 above to determine where the bend is on the line, then use the scan line numbers given in the processing run and a bit of trial and error to cut out the bend using the -l option:

azgcorr ... -l <start_line> <end_line> ...

This will geocorrect those parts of the flight between start_line and end_line. If for example you had a flightline that was 20000 lines long and had a bend from lines 7500 - 7900, you'd use two azgcorr commands:

azgcorr ... -l 0 7500 ...
azgcorr ... -l 7900 20000 ...

This would produce two geocorrected images, each with one part of the line.

Q: Is there any way to speed up geocorrecting my data or to reduce the file size of the geocorrected files?

A: There are three main ways to do this:

  1. Only geocorrect bands you want to look at. Azgcorr can be passed a -bl parameter that tells it only to geocorrect bands in the given list. For example, to correct bands 4, 5 and 6 you'd use:

azgcorr ... -bl 4 5 6 -1 ...

...the -1 at the end is necessary to tell azgcorr that you've finished giving it band numbers.

  1. If you're only interested in part of the flightline, don't process the parts you're not interested in. This may require some trial-and-error to establish the line numbers. Use the -l option to azgcorr to process only a certain range of scan lines. You'll need to run the whole thing through once, optionally with only one band as above, to check the total number of scan lines. For example, for a flightline with 10000 scan lines, if you only want lines 2000-6000 you'd use:

azgcorr ... -l 2000 6000 ...

  1. Make the pixel size you're using larger - larger pixels give less detail, but the file size is inversely proportional to the square of the pixel size, so if you double the pixel size (if you don't need the extra detail) you will reduce file sizes by roughly 75%.