resampling MODIS-based raster to PRISM raster to obtain same extent

Julie,

Yes, in that case the projection (coordinate reference system -- CRS)
will be the same. That is sufficient for vector data (points, lines,
polygons), but not for raster data. For raster data you also need to
match the resolution and origin such that the cells are aligned. With
origin I refer to the location nearest to (0, 0) that the edge of a
particular raster could come if only the extent is changed (resolution
is fixed). If origins are not the same, the rasters are not aligned,
and you cannot directly compare values for matching cells (as cells do
not match). Even if two rasters are aligned, you may still need to
crop/expand one or both such that they get exactly the same extent
(bounding box).  resample can come to the rescue here but it is
preferable to avoid it. See the raster package vignette for more info.

I am under the impression that the MRT does not allow you to set an
origin. If that is true, then it is an inadequate tool for changing
the projection of raster data and I would use GDAL instead. On linux
you can have rgdal with HDF5 support, but not on windows. But on
windows you can use command line GDAL (from FWtools)  perhaps via the
new gdalUtils package.

Robert


On Thu, Jan 16, 2014 at 11:48 PM, Julie Lee-Yaw <julleeyaw at yahoo.ca> wrote:

Hi Robert,

Thanks for the response. I have done a bit more reading but am still
struggling to understand: if I set the datum (NAD83) and the resolution of
the NDVI data to match that of the prism data is that not sufficient for
compatible projections? Does the datum not provide the origin? The extent is
what I am now trying to adjust (e.g. the NDVI data is for all of North
America, the PRISM data is for the US only). Or do you mean something else
by extent?

The prism data has a prj file that reads:

GEOGCS["NAD83",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]

Thanks again for the help!

Julie




On Tuesday, January 14, 2014 8:11:01 PM, Robert J. Hijmans
<r.hijmans at gmail.com> wrote:
Julie,
You raise an important question that is often overlooked. While it is
possible to use resample as you suggest; you would want to avoid it
because it leads to loss of data quality (although in practice this is
often minimal and irrelevant).

You state that you "project it to the same geographic coordinate
system as PRISM precip. data". But that is an incomplete statement for
raster data. What you need to do is to project the NDVI data to the
raster definition used by PRISM. That includes the coordinate system,
but also the origin (or extent) and resolution of that raster. I do
not think MRT supports setting these parameters; in which case you
should not use it. You can use GDAL instead. On Linux this can be done
with rgdal; on windows you can use FWTools instead, or perhaps the new
gdalUtils package?

Robert

On Mon, Jan 13, 2014 at 3:39 PM, Julie Lee-Yaw <julleeyaw at yahoo.ca> wrote:

Hi

Using a combination of the scripts provided here (by Babak N.):
http://r-gis.net/?q=ModisDownload and the MODIS reproject tool, I've finally
managed to download NDVI data for North America and project it to the same
geographic coordinate system as PRISM precip. data (e.g.
http://www.prism.oregonstate.edu) for the same time period.

I now want to stack these two rasters. My NDVI layer has a greater extent
than the PRISM layer so I crop the former by the latter using:

croppedNDVI<-crop(NDVI,prism)


But when I look at the resulting raster, I see that the extent still
doesn't line up. I think this is an issue with cells of the two rasters
being "off" centre from each other. I can use the following to get them to
align:

adjustNDVI<-resample(croppedNDVI,prism)


Now I can stack the "adjustNDVI" and "prism" layers as the extents match.
But I am wondering whether this is valid? Why were the initial rasters
misaligned in the first place given that I specified the same resolution and
geographic coordinate system/datum when I processed the MODIS file? I'm
grateful for any clarification!
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