# [Proj] lat/long to northing/easting and back again

Clifford J Mugnier cjmce at lsu.edu
Mon Mar 16 11:31:23 EST 2009

```I'd like to point out that NADCON and its derivatives do exactly that - the algorithm does a bi-linear interpolation in terms of the graticule.  The ultimate "accuracy" is dependant on the fineness of the "mesh" of the graticule, and not on the interpolation.

C. Mugnier
LSU

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From: proj-bounces at lists.maptools.org on behalf of Christopher Barker
Sent: Mon 16-Mar-09 11:28
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] lat/long to northing/easting and back again

Roger Oberholtzer wrote:
>> First, interpolating positions from easting/northing values is shaky and
>> accuracy of such efforts is highly dependent upon the distance between points
>> used in the operation.

My first thought is -- why so this in projected space? It sounds like
you have lat-long, and you want to interpolate between two points, and
get back lat/long again. So could you do this in lat-long space?

To answer that, we need to know what you mean by interpolate?

Do you have two points and want to know what the midpoint is, for
example? If so, what does "mid point" mean? That's not obvious on a
globe, which may be why you're using projected coordinates, but with
Gerald's recently released geodetic (geodesic?) tools, it should be
doable to find a point along a great circle connecting two points, for
instance.

> The distance is in the order of meters. Typically less than 10.

In that case, you can probably get pretty darn close by pretending that
that lat-long units are Cartesian -- and that would be really easy! How
accurate do you really need to be?

-Chris

--
Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
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