[R-sig-Geo] Unexpected elevation value when using extract() on RasterLayer

[Matthew Nelsen]

Hi Everyone,

I'm fairly new to using R to analyze geo data and am getting some
unexpected values when I try to extract elevation data from a RasterLayer
using lat/long.  I've downloaded some global elevation data (ESRI ArcView
Format) from here:

http://nelson.wisc.edu/sage/data-and-models/atlas/maps.php?datasetid=28&includerelatedlinks=1&dataset=28

And have read it in using raster and rgdal.  I then am trying to extract()
the elevation at specific lat/longs and have been getting some strange
values.  For instance, I try to get the elevation for a point in Samoa
(-13.86472, -171.77.06), and an elevation of -129m is returned, which is
incorrect.  Any idea why this might be?  I'm not sure where I'm going wrong
(or if this specific RasterLayer does not deal well with smaller land
masses and truly does say it is -129m)? I've tried it two different ways
(below).

Thanks very much for any help.

All the best,

-Matt.


#download elevation data from:
http://nelson.wisc.edu/sage/data-and-models/atlas/maps.php?datasetid=28&includerelatedlinks=1&dataset=28

#click Download a GIS grid of this data (ESRI ArcView Format)


ll<-cbind(-171.7706,-13.86472)

#coordinates are inland in Samoa

#https://www.google.com/maps/place/13
°51'53.0%22S+171°46'14.2%22W/@-13.86472,-171.8034302,13z/data=!4m5!3m4!1s0x0:0x0!8m2!3d-13.86472!4d-171.7706


require(raster)

#read hdr.adf file

elev<-raster("/...PATH.../elevation/elevation/hdr.adf")

elev


#class       : RasterLayer

#dimensions  : 2160, 4320, 9331200  (nrow, ncol, ncell)

#resolution  : 0.08333333, 0.08333333  (x, y)

#extent      : -180, 180, -90, 89.99999  (xmin, xmax, ymin, ymax)

#coord. ref. : NA

#data source :
/...PATH.../atlas_of_the_biosphere_13sep2017/elevation/elevation/hdr.adf

#names       : hdr

#values      : -10376, 7320  (min, max)

#attributes  :

#           ID COUNT

# from: -10376     1

# to  :   7320     1



extract(elev,ll)

#-129


####Alternatively, read in...

require(rgdal)

#from
https://gis.stackexchange.com/questions/132403/how-to-read-adf-files-into-r

dpath<-"/...PATH.../elevation/elevation"

x<-new("GDALReadOnlyDataset",dpath)

getDriver(x)

#An object of class "GDALDriver"

#Slot "handle":

#<pointer: 0x60000032ac80>


getDriverLongName(getDriver(x))

#[1] "Arc/Info Binary Grid"


xx<-asSGDF_GROD(x)

elevation <- raster(xx)

elevation


#class       : RasterLayer

#dimensions  : 2160, 4320, 9331200  (nrow, ncol, ncell)

#resolution  : 0.08333333, 0.08333333  (x, y)

#extent      : -180, 180, -90, 89.99999  (xmin, xmax, ymin, ymax)

#coord. ref. : NA

#data source : in memory

#names       : band1

#values      : -10376, 7320  (min, max)


extract(elevation,ll)


#-129

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