reading .hdf files

Yes, I checked the projection info from the newly converted tiff file, and it
is blank. I even tried to set the projection with the command below to match
my poly file;

projection(rast) <- "+proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0"

but this isn't right. I tried to use the GDALinfo command to extract the
metadata, but that doesn't work either. The ftp site I downloaded the .hdf
files from is run by the NOAA, and I found some info;

Table 2 Description of File attributes (Meta data)
Meta data tag   Content
GRID_ROWS       Number of rows for data grid
GRID_COLUMNS    Number of columns for data grid
START_LATITUDE_RANGE    The latitude of the grid's northern edge
END_LATITUDE_RANGE      The latitude of the  grid's southern edge
START_LONGITUDE_RANGE   The longitude of the grid's western edge
END_LONGITUDE_RANGE     The longitude of the  grid's eastern edge
PROJECTION      Projection type

But how do I view the metadata?



Jonathan Greenberg-3 wrote

I want to follow up with this -- I confirmed Barry's suspicion that
that file you sent above ("GVIX_NL_G16_C07_NVI_Y2005_P26.hdf") has no
projection information (or bounding box or pixel resolution), so there
is no way to do the extract unless that is fixed.  Run a:

gdalinfo("GVIX_NL_G16_C07_NVI_Y2005_P26.hdf")

and you'll see there is not a properly set projection -- the .tif,
thus, also doesn't have any projection info.  This is an issue with
the input file, not with any of our functions.  You will need to set
your own projection, bounding box, and pixel sizes on the raster via
e.g. ?projection, ?res, ?extent

What are these files?  Are they satellite data?  Do you know if there
is another source for them that may have the information set
"properly"?

--j

On Fri, Mar 28, 2014 at 11:38 AM, Barry Rowlingson
<

b.rowlingson at .ac

> wrote:

On Fri, Mar 28, 2014 at 4:11 PM, dododave <

dododave@

> wrote:

I am now able to convert the hdf files into GeoTiffs. I have a follow-up
question though.

I now need to extract the mean ndvi value from within a polygon overlaid
on
that GeoTiff. The issue I am having is that the code is returning NULL
when
I do.

      rast <- raster("2011_P01.tif")
      mean.ndvi <- extract(rast, poly, fun=mean)
mean.ndvi

[[1]]
NULL


Here is the output when I call rast;

rast

class       : RasterLayer
dimensions  : 904, 2500, 2260000  (nrow, ncol, ncell)
resolution  : 1, 1  (x, y)
extent      : 0, 2500, 0, 904  (xmin, xmax, ymin, ymax)
coord. ref. : NA
data source : /Volumes/GOLONDRINA/tiff/2011_P01.tif
names       : X2011_P01
values      : -32768, 32767  (min, max)

Help appreciated!

Are you sure your polygon is in the right place?

r=raster()
r[]=runif(180*360)
pts=data.frame(x=c(-50,50,50,-50,-50),y=c(-50,-50,50,50,-50))
poly = SpatialPolygons(list(Polygons(list(Polygon(pts)),ID=1)))
extract(r,poly,fun=mean)
[1] 0.4997085

 - so that;s all good, but if I move the polygon way off the raster:

poly2 = SpatialPolygons(list(Polygons(list(Polygon(pts+200)),ID=1)))
extract(r,poly2,fun=mean)
[[1]]
NULL

 I get what you're getting.  Check your polygon coordinates and CRS
string
is correct...

Barry

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R-sig-Geo@

--
Jonathan A. Greenberg, PhD
Assistant Professor
Global Environmental Analysis and Remote Sensing (GEARS) Laboratory
Department of Geography and Geographic Information Science
University of Illinois at Urbana-Champaign
259 Computing Applications Building, MC-150
605 East Springfield Avenue
Champaign, IL  61820-6371
Phone: 217-300-1924
http://www.geog.illinois.edu/~jgrn/
AIM: jgrn307, MSN:

jgrn307@

, Gchat: jgrn307, Skype: jgrn3007

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