Hi,
I am new to doing spatial analysis. I am interested in trying many things
in R, but one of the first things I would like to do is plot all of my lat,
lon points in a csv file within a window defined by a shapefile.
I have read in the shape file by doing the following:
border <-readShapePoly("shapefiles/maryland.shp")
plot(border, border="red", las=1) # plot confirms correct shape
border <-as(border, "owin")
However, once I try to do:
data_ppp <-ppp(data$Latitude, data$Longitude, window=border)
I get the error that:
In ppp(data$Latitude, data$Longitude, window = border) :
523461 points were rejected as lying outside the specified window
I realized that the coordinates are completely different. The coordinates
in my data file (i.e. my csv file) are traditional GPS coordinates (e.g. 39.17
or 76.37). The shapefile however has x,y values such as 1416813.54262877
or 561125.546602725. I am not familiar with what type of coordinate system
those values use.
How can I change the coordinates in my csv file to match the same system as
my shapefile or the other way around--i.e. to get my shapefile to have
similar coordinates as my csv file?
Any help would be appreciated.
Thank you.
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On Thu, Jun 13, 2013 at 6:26 AM, L S <losedaghat at gmail.com> wrote:> I realized that the coordinates are completely different. The coordinates > in my data file (i.e. my csv file) are traditional GPS coordinates (e.g. 39.17 > or 76.37). The shapefile however has x,y values such as 1416813.54262877 > or 561125.546602725. I am not familiar with what type of coordinate system > those values use. > > How can I change the coordinates in my csv file to match the same system as > my shapefile or the other way around--i.e. to get my shapefile to have > similar coordinates as my csv file?You really need to know the coordinate reference system that your points are in. What you call "traditional GPS coordinates" is formally known as EPSG:4326. I suspect your points are in some small-region plane coordinate system. In the UK, for example, we have a square metric grid system that is EPSG:27700. The units of this system are metres from a point origin off the south-west of the UK, which is great for small areas in the UK but not applicable to India, because it assumes a flat earth. If your data is in the USA, then it might be one of the state plane systems. There and elsewhere, it might be one of the global UTM zones (but you have to know which one). There are thousands of possible coordinate systems: http://www.epsg-registry.org/ or you can make your own using PROJ4 strings... To convert from one coordinate system to another, I normally use spTransform from the rgdal package since I mostly work with sp-class objects. I think there's a 'project' function that can work with raw x-y coordinates in vectors or matrices. You can also try asking on the R-sig-geo mailing list where the map people hang out. Barry
Your shapefile should include specifications for its coordinate system. Look for a file names maryland.prj in the same directory as maryland.shp. And that coordinate system should be included in your 'border' object. You might need to change to using readOGR() to load the shapefile into R. Use str(border) to find out. If your border object does in fact include its projection, then you can use spTransform(), as Barry suggested, to transform it to lat/long. Then your overlay should succeed. And I second Barry's recommendation to ask this kind of question on r-sig-geo. You will need to learn how to specify coordinate systems; a starting point is ?proj4string (from the sp package). r-sig-geo is definitely a good place to get help for that. I myself have used QGIS (Quantum GIS) to look up projection specs, as it has a nice listing which you can search by name; in your case I'd search for 'Maryland'. -Don -- Don MacQueen Lawrence Livermore National Laboratory 7000 East Ave., L-627 Livermore, CA 94550 925-423-1062 On 6/12/13 10:26 PM, "L S" <losedaghat at gmail.com> wrote:>Hi, > >I am new to doing spatial analysis. I am interested in trying many things >in R, but one of the first things I would like to do is plot all of my >lat, >lon points in a csv file within a window defined by a shapefile. > >I have read in the shape file by doing the following: >border <-readShapePoly("shapefiles/maryland.shp") >plot(border, border="red", las=1) # plot confirms correct shape >border <-as(border, "owin") > >However, once I try to do: >data_ppp <-ppp(data$Latitude, data$Longitude, window=border) > >I get the error that: >In ppp(data$Latitude, data$Longitude, window = border) : > 523461 points were rejected as lying outside the specified window > >I realized that the coordinates are completely different. The coordinates >in my data file (i.e. my csv file) are traditional GPS coordinates (e.g. >39.17 >or 76.37). The shapefile however has x,y values such as 1416813.54262877 >or 561125.546602725. I am not familiar with what type of coordinate system >those values use. > >How can I change the coordinates in my csv file to match the same system >as >my shapefile or the other way around--i.e. to get my shapefile to have >similar coordinates as my csv file? > >Any help would be appreciated. > >Thank you. > > [[alternative HTML version deleted]] > >______________________________________________ >R-help at r-project.org mailing list >https://stat.ethz.ch/mailman/listinfo/r-help >PLEASE do read the posting guide >http://www.R-project.org/posting-guide.html >and provide commented, minimal, self-contained, reproducible code.