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

[Sarah Goslee]

On Thu, Aug 28, 2014 at 9:32 AM, Michael Sumner <mdsumner at gmail.com> wrote:
> 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.

True, but as long as your points are reasonably close together,
something like UTM is a very useful approximation. And even great
circle is an approximation. The best answer depends on the data and
the objectives (as always!).

> 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