[R-sig-Geo] Verify units of distance between coordinates

[Michael Sumner]

On Thu, Aug 28, 2014 at 10:50 PM, Sarah Goslee <sarah.goslee at gmail.com> wrote:
> They don't make sense.
>
> Best: convert them into a projection where the distances are in meters
> already, like UTM. Then distances calculated on your new coordinates
> are in meters.

However great circle from lat/lon is arguably the best since you can
really do get distance along a great circle (on the ellipsoid or the
sphere). (There are several algorithms, and also other methods for
e.g. loxodromes, and even other definitions of "straight".)

No projection has the property that any straight line is a great
circle, and most certainly *not* any of UTM family.

Cheers, Mike.


>
> Latitude and longitude don't translate neatly into distances on their own.
>
> Second best: find and use a great circle distance function that can
> determine the correct distances for where those lat/lon coordinates
> are on the earth's surface. There's been discussion on this list
> before about calculating distances from geographic coordinates; google
> should find them.
>
> Sarah
>
> On Thu, Aug 28, 2014 at 8:44 AM, Justin Michell <jwm302 at gmail.com> wrote:
>> Dear geo R group
>>
>> I have a data frame like this:
>>
>> df <- data.frame(Lon = c(29.6000,29.7333,30.3887,30.6667,30.6833,30.8667), Lat = c(-4.9000,-4.6000,-5.1280,-1.0667,-2.7500,-3.3833),
>>                   LonWater = c(29.63333,29.63333,30.25000,30.65000,30.35444,30.83278), LatWater = c(-4.31667,-4.31667,-4.76667,-1.35000,-2.46667,-3.57000), DstClW = c(0.5842815,0.3004491,0.3870362,0.2837918,0.4340793,0.1897561) )
>>
>> At these locations (Lon, Lat pairs) I calculated the shortest distance to a water source (DstClW) and where that source is (LonWater, LatWater).
>>
>> I want to now determine what units DstClW is in, and also verify that these distances make sense and were calculated correctly.
>>
>> Any suggestions as to how this might be done?
>>
>> Regards
>> Justin Michell
>>
>>
>>
> --
> Sarah Goslee
> http://www.functionaldiversity.org
>
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-- 
Michael Sumner
Software and Database Engineer
Australian Antarctic Division
Hobart, Australia
e-mail: mdsumner at gmail.com