projecting Lat and Long to 2D
If your sites are more or less all over the world, why do you want to project? The Earth is not flat. If this is just for cartographic display, you can use goode homolosine for example (http://en.wikipedia.org/wiki/Goode_homolosine_projection). But for calculating distances, the most reasonable is using geodesic distance. Agus
Harry Kim wrote:
Dear R-sig-Geo listers,
These are very newbie like questions but if i could get any kind of
help, i would very much appreciate it.
I am trying to project points encoded in latitude and longitude to
hopefully Cartesian coordinates with a reasonable unit of distance
(meters or kilometers.) When i encountered this problem previously,
i've used PBSmapping package and convUL() to convert the points to
UTM coordinates.
However, the points that i wish to project now are all over eurasia
and Africa. I feel that if i just naively convert all the points to
UTM, then i will run into a trouble as the points have different UTM
zones. Is this correct? If i have some points in zone 10 and zone 14,
and wish to find the distance between them using UTM coordiates and
euclidean norm, would it provide a reasonable approximation of their
distance? If not, is there a projection you would recommend that will
give me a reasonable results?
I would also like to overlay the points with a world map excluding
America and Antartica. I know that i can plot the map of the world
using maps package and map("world",regions="") but i don't know how i
would exclude the two continents.
I tried to export the world map into a shapefile then read it in to R
using readShapPoly() in maptools package, then change the coordinate
system into UTM using spTransform, but this ended up giving me a very
weird looking figure.
In sum, i would like to know if there is a good way to project these
points and overlay a map such that i can use euclidean norm to find
the distance, and the map won't look completely bizzare. Also i would
like to know if i could exclude the two continents mentioned: the
America and the Antarctica.
I thank you very much in advance and i hope you are having a pleasant day.
Harry
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Dr. Agustin Lobo Institut de Ciencies de la Terra "Jaume Almera" (CSIC) LLuis Sole Sabaris s/n 08028 Barcelona Spain Tel. 34 934095410 Fax. 34 934110012 email: Agustin.Lobo at ija.csic.es http://www.ija.csic.es/gt/obster