[R-sig-Geo] strange srtm3 query in geonames package

[Michael Denslow]

Thanks Barry!

On Sun, Nov 22, 2009 at 2:02 PM, Barry Rowlingson
<b.rowlingson at lancaster.ac.uk> wrote:
> On Sun, Nov 22, 2009 at 6:23 PM, Michael Denslow
> <michael.denslow at gmail.com> wrote:
>> Dear r-sig-geo and Barry,
>>
>> I am getting a strange value when doing a query using the function
>> GNsrtm3 in the geonames package.
>> Here is the code.
>>
>> library(geonames)
>> GNsrtm3(34.63874,-79.10111)  # outside Lumberton North Carolina USA!
>>
>>
>>   srtm3       lng      lat
>> 1 -32768 -79.10111 34.63874
>>
>> Warning message:
>> In readLines(u) :
>>  incomplete final line found on
>> 'http://ws.geonames.org/srtm3JSON?lat=34.63874&lng=-79.10111'
>>
>>
>> I think the elevation value should be around 47 meters. The geonames
>> website says 'ocean areas have been masked as "no data" and have been
>> assigned a value of -32768'.
>> http://www.geonames.org/export/web-services.html#srtm3
>> I tried adjusting the lat, long values to query some nearby cells but
>> I still get -32768.
>>
>> Are these kinds of errors common in this dataset? Any suggestions?
>> I am would actually prefer to get an average elevation for the county
>> (Robeson) but am settling for the county centroid at this point.
>
>  I don't really know the details of the srtm3 data set, but I do see
> that geonames has two other DEM datasets - ASTER and GTOPO30. There's
> a function for GTOPO30 in geonames, but not ASTER. It's easy enough to
> write though:
>
>> GNaster=function(lat,lng){return(as.data.frame(geonames:::getJson("astergdemJSON",list(lat=lat,lng=lng))))}
>
>  and I may include it in a new release (as well as getting rid of
> those end-of-line warnings). These two services agree with themselves
> and with you pretty well for your test location:

It would be great to have GNaster added.

>
>> GNgtopo30(34.63874,-79.10111)
>  gtopo30       lng      lat
> 1      46 -79.10111 34.63874
>
>> GNaster(34.63874,-79.10111)
>  astergdem       lng      lat
> 1        47 -79.10111 34.63874
>
>  For an 'average' elevation you might be better off sampling many
> locations over your polygon.

Sounds like a good idea.

Thanks again for your help!

Michael

>
> Barry
>



-- 
Michael Denslow

Graduate Student & Adjunct Instructor
I.W. Carpenter Jr. Herbarium [BOON]
Department of Biology
Appalachian State University
Boone, North Carolina U.S.A.
-- AND --
Communications Manager
Southeast Regional Network of Expertise and Collections
sernec.org

36.214177, -81.681480 +/- 3103 meters