[GRASS-user] Alaska shapefile: Convert from -180,180 to 0-360

[Paul Van Deusen]

Thanks for the feedback. I ended up converting longitude to 0-360 as 
Brent suggested. Its fairly easy to
do with R. I included the little R script that I wrote in case anyone 
else might find it useful. In general, R
is good for manipulating shapefiles.

======R script follows============

##Use this script to pull a state out of the national shapefile that came
##from http://nationalatlas.gov. It also converts lon from
##-180,+180 to 0-360, and also demonstrates how to work with shapefiles

#boundingBox returns a matrix of points representing a bounding box of a 
submitted user polygon
boundingBox <- function(xy){
yMin <-min(xy[,2]); yMax <-max(xy[,2]);
xMin <-min(xy[,1]); xMax <-max(xy[,1]);
bb<-c(xMin,yMin, xMax, yMax);
return(bb);
}


library(maptools) #load the library
fName="statesp020"; #name of shapefile that needs sorting and trimming
fOut="02"; #output file name, AK fips code=02
ST="Alaska" #state name in dbf file

shp <- read.shape(paste(fName,".shp",sep="")) #read in the original 
shapefile
dbf <- read.dbf(paste(fName,".dbf",sep="")) #read in the original dbf file

cat("Wait while making a polyList \n")
shapes <- Map2poly(shp,region.id=NULL)

keep=dbf[,"STATE"]==ST

sub=subset(shapes,keep)

dbf2 = dbf[keep,]

#convert from +180,-180 to 0-360 (only needed for Alaska)
for(i in 1:length(sub)){
vec=sub[i][[1]][,1]<0;
sub[i][[1]][vec,1]= 360 + sub[i][[1]][vec,1]
attr(sub[i][[1]],"bbox") = boundingBox(sub[i][[1]])
centroid=attr(sub[i][[1]],"centroid")$x
if(centroid<0)attr(sub[i][[1]],"centroid")$x=360+centroid
}


write.polylistShape(sub,dbf2,fOut,factor2char = TRUE)

cat("\n its done \n")

============end of R script=================

Gary Sherman wrote:
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> Hash: SHA1
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> If you are only interested in Alaska an perhaps part of Canada and 
> Russia, you could always project the data to Alaska Albers:
> - -PROJ_INFO-------------------------------------------------
> name : Albers Equal Area
> proj : aea
> datum : nad83
> a : 6378206.4
> es : 0.006768657997291279
> lat_1 : 55
> lat_2 : 65
> lat_0 : 50
> lon_0 : -154
> x_0 : 0
> y_0 : 0
> no_defs : defined
> - -PROJ_UNITS------------------------------------------------
> unit : meter
> units : meters
> meters : 1
>
>
> On Mar 15, 2007, at 1:43 PM, Brent Wood wrote:
>
>> Paul Van Deusen wrote:
>>
>> Welcome to my world :-)
>>
>> Working in the maritime area around New Zealand, 180 is an issue I 
>> deal with regularly :-(
>>
>> I suggest you add 360 to every longitude < 0. This moves the break to 
>> zero degrees instead of 180.
>>
>> You can also use a variety of local projections to avoid the issue.
>>
>> There is a parameter in the CVS version of proj4 to set the central 
>> meridian, defaults to zero, but a reprojection to WGS84 (EPSG:4326) 
>> with explicit parameters, & the central meridian of 18- works well....
>>
>> Brent Wood
>>
>>> I'm trying to read in the state boundary shapefile from 
>>> http://nationalatlas.gov. However, the Aleutian Islands
>>> cross the 180th parrallel, so longitude goes from +180 to -180 at 
>>> that point. I end up with the end of the
>>> Aleutian Islands getting disconnected. What is the solution? I used 
>>> v.in.ogr and set up a location using the information that came
>>> with the shapefile. See spatial information below..
>>>
>>> Spatial_Reference_Information:
>>> Horizontal_Coordinate_System_Definition:
>>> Geographic:
>>> Latitude_Resolution: 0.000278
>>> Longitude_Resolution: 0.000278
>>> Geographic_Coordinate_Units: Decimal degrees
>>> Geodetic_Model:
>>> Horizontal_Datum_Name: North American Datum of 1983
>>> Ellipsoid_Name: GRS1980
>>> Semi-major_Axis: 6378137
>>> Denominator_of_Flattening_Ratio: 298.257222
>>>
>>> _______________________________________________
>>> grassuser mailing list
>>> grassuser at grass.itc.it
>>> http://grass.itc.it/mailman/listinfo/grassuser
>>
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>
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> Gary Sherman
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-- 
Paul Van Deusen
NCASI
600 Suffolk St. Fifth Floor
Lowell, MA 01854
978-934-1948