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I am using the incredible MODISDownload function from the rts package in R to download NDVI data on vegetation index in Africa.

I have two questions.

I am using the latest version of the package available on github, and I download and reproject the images with the following code (which works beautifully):

ModisDownload(x=x,h=c(19),v=c(5), version = '006', dates=c('2000.01.01', '2000.02.28'),
              MRTpath=path,mosaic=F,proj=T, bands_subset="1 0 0 0 0 0", 
              proj_type="GEO",proj_params="0 0 0 0 0 0 0 0 0 0 0 0 0 0 0",
              datum="WGS84",pixel_size=0.001)

where x = "MOD13Q1" and path is the path to my MRT/bin folder. I choose only band 1 because I am interested in the NDVI index, which is the first band. I choose proj_type = "GEO" as I want the projection on longitude/latitude format.

I am not very sure about my choice of the proj_params and pixel_size parameter. For the former, I mimic what is automatically set if I do the reprojection using the MRT GUI interface, but I am not sure whether that is correct.

For the pixel_size, though, I am completely lost. It is clear that the smallest its values, the higher the resolution of the outcome tif file. However, the higher will the its size on disk. Thus, I was wondering whether someone would know how can I calculate an 'optimal' pixel size for the 250m resolution.

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Always check reference manual. See Apendix C to know about projection parameters. They are used for datum conversions:

enter image description here

If you want a 250m pixel size, use a meter projected CRS like WGS 84 UTM zone X n/s and set pixel size = 250. Working with a lat/long projection isn't possible to get a 250m pixel size for all locations.

  • That is exactly what I was looking for. Thank you so much! – Bruno Conte Leite Dec 12 '17 at 10:10

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