[R-sig-Geo] generating coordinates at equal geographical distances

[Michael Sumner]

There are many pathways to this, but you could use a map projection
that provides the properties you want and sample points there, then
project that back to longitude latitude.

Here's a dummy example, I just found an equal area projection by
stumbling through spatialreference.org, you should look into this more
for your purposes. I use spsample in the sp package to generate the
points, but what you use here really depends on what kind of points
you want, I ignored land vs. sea but you could easily adapt this to
the land only.

library(maptools)

data(wrld_simpl)
w <- subset(wrld_simpl, NAME %in% c("Canada", "Mexico", "United States"))
library(rgdal)
proj <- "+proj=laea +lat_0=45 +lon_0=-100 +x_0=0 +y_0=0 +a=6370997
+b=6370997 +units=m +no_defs"
w <- spTransform(subset(wrld_simpl, NAME %in% c("Canada", "Mexico",
"United States")), CRS(proj))

## create an extent object to sample from, rather than just the land
## (just use the "w" object if you want the points to be only on land)
library(raster)
bbpoly <- as(extent(w), "SpatialPolygons")
proj4string(bbpoly) <- CRS(proj4string(w))

x <- spsample(bbpoly, n = 2000, type = "hexagonal")
pts <- spTransform(x, CRS(proj4string(wrld_simpl)))
plot(pts)
plot(as(wrld_simpl, "SpatialLines"), add = TRUE, col = "dodgerblue")


On Thu, Aug 29, 2013 at 3:27 AM, john d <dobzhanski at gmail.com> wrote:
> Ok, I'll be more explicit :)
>
> If I want to generate random coordinates across North America by uniformly
> sampling latitudes and longitudes, the geographical distance between these
> coordinates would decrease towards higher latitudes. I'd like to be able to
> generate coordinates (e.g. a grid) such that the geographical distances
> between coordinates would be constant regardless of latitude.
>
>
>
> On Wed, Aug 28, 2013 at 12:04 PM, Barry Rowlingson <
> b.rowlingson at lancaster.ac.uk> wrote:
>
>> You will find that any two coordinates are equidistant from each other...
>>
>>  So my suggestion is you need to expand you question a bit more! :)
>>
>> On Wed, Aug 28, 2013 at 3:49 PM, john d <dobzhanski at gmail.com> wrote:
>> > Dear all,
>> >
>> > I need to generate coordinates that are equidistant, regardless of
>> > latitude. Any suggestions?
>> >
>> > Thanks!
>> >
>> > John
>> >
>> >         [[alternative HTML version deleted]]
>> >
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>>
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-- 
Michael Sumner
Hobart, Australia
e-mail: mdsumner at gmail.com