[GRASS-user] GRASS 7 subpixel shift of imported Landsat data

[Nikos Alexandris]

* Tomáš Brunclík <brunclik at atlas.cz> [2015-05-27 12:49:13 +0200]:

> Hello all,
> working with GRASS GIS 7.0.1svn (r65198) @Linux, I noticed, that when 
> importing two Landsat images from different dates, I often get them 
> spatially shifted to each other when comparing them in detail - meaning 
> when you zoom in to about 1:5000 scale and switch the viewed bands, the 
> features in the map jump by a fraction of pixel size. For Landsat 8 
> images LC81910252013208 (reflectance product) and LC81910252013224 (DN 
> product) the shift is about 15m (half the pixel size) in both horizontal 
> and vertical direction. The same two products imported in GRASS 6.4.4 
> have no visible shift at all and the corresponding pixels seem to be 
> perfectly aligned.
> 
> The procedure to reproduce this:
> 
> 1)Download and uncompress this archive (133 MB):
> https://dl.dropboxusercontent.com/u/24062833/pixel_shift_208_224.zip
> /
> The archive contains band 5 of the two above mentioned Landsat scenes, 
> where the one with DOY 208 (the last number of the scene code, 
> day-of-year) is from surface reflectance (SR) product, the other with 
> DOY 224 is digital number (DN) product (I work on a script to 
> atmospherically correct DN product based on reference SR image)//.//
> /
> 
> 2) Start GRASS 7, create new mapset in location with UTM 33N 
> (EPSG:32633) spatial reference system.
> 
> 3) Import each image, open them in the map, set colors, zoom in and 
> compare a distinct feature placement in detail:
> 
> #import
> r.in.gdal input="/path/to/data/LC81910252013208LGN00_sr_band5.tif" 
> output=208_grass7
> r.in.gdal input="/path/to/data/LC81910252013224LGN00_B5.TIF" 
> output=224_grass7
> 
> #set the null pixels of the DN band (helps the bands to get similar 
> color palette in the next step)
> r.null map=224_grass7 setnull=0
> 
> #set colors:
> r.colors map=208_grass7
> r.colors map=224_grass7
> 
> 
> 4) Zoom in to a distinct feature in about 1:5000 scale (I have used a 
> pond at coordinates 542365,554675), switch visibility of both images on, 
> then repeatedly switch on/off the upper image in the stack. You should 
> see the shift quite clearly. Zooming in even more, it is possible to 
> measure the shift.
> 

Tomas,

I had a quick look. Pixels are perfectly aligned here (after going
through all of the steps you mention here) in G7.

Which version of GDAL are you on?

Nikos

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