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

Roger Oberholtzer roger at opq.se
Mon Mar 16 13:24:09 EST 2009

```On Mon, 2009-03-16 at 09:28 -0700, Christopher Barker wrote:
> 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?

We need to involve the odd gyroscope, inclinometer, and pulse distance
transducer at this point. All are calculating distances in meters. That
could probably be changed. If it was a GPS-only solution, then the
question would not have to be asked.

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

Our data is tagged with time that has been synchronized with our GPS.
When we want to know where we are, we start by determining when we were
at the point we want to know the location of.

> 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 distances are luckily too small to have to involve the great circle.

> > 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
>
>
--
Roger Oberholtzer

OPQ Systems / Ramböll RST

Ramböll Sverige AB
Krukmakargatan 21
P.O. Box 17009
SE-104 62 Stockholm, Sweden

Office: Int +46 8-615 60 20
Mobile: Int +46 70-815 1696

```