[GRASS-user] value differences between landsat images

[Hamish]

Juan Manuel Barreneche:
> This may be a very technical question, but i hope somebody
> knows the answer
> 
> I'm currently working with landsat 5 images, trying to
> identify water
> bodies with an algorithm that i found on the web (*). The
> thing is, many calculations are made in the process, including NDVI
> and normalization of the different bands, but there are
> important differences between landsat images, even with adjacent
> ones. This differences in values can be > 30, for the same kind of
> terrain, e.g.:
> 
> r.what -f input=223_082_B
> east_north=502773.212142,6416651.23944
> 502773.212142 | 6416651.23944 || 83 |
> 
> r.what -f input=223_083_B
> east_north=502789.528784,6415683.11873
> 502789.528784 | 6415683.11873 || 49 |
> 
> (in this example i used the r.what function in both images,
> in some area that is repeated but slightly moved from one image to
> the other)
> 
> This would obviously cause a difference in the calculations
> that i want to run, and so can be a big problem for me, but that
> depends of
> how these differences are produced, for which i imagine two
> scenarios:
> 1. In one hand, if the different values are generated by a constant
> that multiplies all the values in the image (i.e., all the
> "real" values), then the problem is solved, because said constant
> would be eliminated in the calculations (ej. in the NDVI formula, 
> it would be divided by itself).

> 2. On the other hand, if the different values are generated
> by a constant that is summed to all the values, then i'm
> really lost, because i can't imagine a way around to solve the
> problem.
> 
> Maybe some other scenario is really ocurrying (maybe a
> combination of both??). I made a small test with two adjacent
> images (by comparing many points like in the example), and it
> looked like the first scenario, but with great uncertainty....
> (In practice i can't do the
> same exploration for all the images that i have, because of
> the error involved and the time that consumes)
> 
> If anyone have any clue that can help i'd be very
> grateful,


are the images taken at the same time of day/year? if not it will be hard to get them to match well as they will be different animals.

if it is simply an "automatic contrast" adjustment, you could linearly interpolate between bands so they matched. (but then which is correct?)
I don't think it would be though, as LANDSAT has fixed calibration for each band.


if you run i.landsat.rgb on the two images with the same parameters do they match up well? that doesn't touch the values, only the colors, but it may give you a clue about the cause of the difference.

I guess the important thing is the ratio of the bands, not the exact values  of one particular band. I take it you see a hard line at the boundary in the processed image?


Hamish