[R-sig-Geo] Distance between two points

[Sarah Goslee]

Hi Simon,

I've copied this back to the list, as is encouraged.

On Fri, Dec 7, 2012 at 9:41 AM, O'Hanlon, Simon J
<simon.ohanlon at imperial.ac.uk> wrote:
> Hi Sarah, thank you.
>
> My data points are located in West Africa, so I think a good projection would be Lambert Azimuthal Equal Area.
>
> This does raise one more question for me. I also have some raster data that I wish to use as covariates which is also in geographic coordinates. If I try to simply reproject a raster, I think it would screw up the regular grid. Does it make sense if I convert the lat-long raster to a SpatialPointsDataFrame and transform this to LAEA so I could then create a grid in the desired planar coordinates from scratch and use over() to assign values from the reprojected spatial points onto the raster?

A reprojected raster is still a raster, and thus a regular grid. Or am
I missing something?

Sarah


> Thanks again for your help.
>
> Cheers,
>
> Simon
>
> -----Original Message-----
> From: Sarah Goslee [mailto:sarah.goslee at gmail.com]
> Sent: 07 December 2012 14:18
> To: O'Hanlon, Simon J
> Cc: r-sig-geo at r-project.org
> Subject: Re: [R-sig-Geo] Distance between two points
>
> Precisely. You should use great-circle distances with lat-lon coordinates, rather than Euclidean distance, because the actual length varies with position on the globe.
>
> Converting to UTM or something similar is one solution if your points are not too far apart.
>
> There are many other R solutions: searching for "great circle distance" at rseek.org will get you quite a list.
>
> Sarah
>
> On Fri, Dec 7, 2012 at 8:10 AM, O'Hanlon, Simon J <simon.ohanlon at imperial.ac.uk> wrote:
>> Dear list,
>> I am using the package geoRglm to do some predictive mapping. There is a function that calculates the distance between observed data points and the prediction locations using a .C call to a function which eventually calculates the length of the hypotenuse between one location and the other given the vertical and horizontal separation distance of those points.
>>
>> My question is, is this method of distance-finding incompatible with long-lat style coordinates? Should I first transform my data and prediction locations into something where the unit of measurement is in metres rather than decimal degrees?
>>
>> Many thanks,
>>
>> Simon
>>


--
Sarah Goslee
http://www.functionaldiversity.org