CODEWITHSUNDEEP
rrasterr-raster
Simon
Simon

Reputation: 1111

R Raster Merge Changes Values

I have a series of GTiff images that I am trying to merge into a single larger extent. 6 small tiles need to be combined to generate my larger extent. My original 6 tiles have values which range from 0 to 255.

For example:

> tiff.list[[1]]
class       : RasterLayer 
dimensions  : 1200, 1200, 1440000  (nrow, ncol, ncell)
resolution  : 926.6254, 926.6254  (x, y)
extent      : -10007555, -8895604, 2223901, 3335852  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=sinu +lon_0=0 +x_0=0 +y_0=0 +a=6371007.181 +b=6371007.181 +units=m +no_defs 
data source : D:\Scratch\Data\MOD15A2.A2016153.h09v06.005.2016166083754.tif 
names       : MOD15A2.A2016153.h09v06.005.2016166083754 
values      : 0, 255  (min, max)

However, when merging the tiles using the code detailed here, I get a new image file and the values have changed:

> xx
class       : RasterLayer 
dimensions  : 2400, 3600, 8640000  (nrow, ncol, ncell)
resolution  : 926.6254, 926.6254  (x, y)
extent      : -10007555, -6671703, 1111951, 3335852  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=sinu +lon_0=0 +x_0=0 +y_0=0 +a=6371007.181 +b=6371007.181 +units=m +no_defs 
data source : D:\Scratch\Modis\A2016161.tif 
names       : A2016161 
values      : 0, 25  (min, max)

Does anyone know why this is happening? I've tried changing the file format and dataType ('INT1U') but it keeps happening. It's important the values don't change from 0 to 255 as the original data comes from NASAs MODIS satellite and certain values (i.e. 248-255) have specific fill values associated with them (for example, land cover assigned as water or snow). This change from a max value of 255 to 25 is removing important information from the original files.

Any assistance provided would be most welcome.

Upvotes: 2

Views: 563

Answers (1)

Robert Hijmans
Robert Hijmans

Reputation: 47481

This suggests that these values are absent in the original files. The min and max values reported for the original files are based on the metadata provided therein. The metadata was likely wrong (showing the range of possible, not the actual values). To investigate do 

setMinMax(tiff.list[[1]])

or 

tiff.list[[1]] * 1

Upvotes: 1

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