Re: OS 50k versus 25k accuracy?

On Thu, 30 Apr 2009 15:14:38 +0100, "Paul Saunders"
<pvs1@xxxxxxxxxxxxxxxxxxxxx> wrote:

Alan Murphy wrote:

Since different BNG grid refs are being shown on the GPS and in
GPSU, that indicates that the datum conversion is different. Perhaps
the Memory Map conversion is more accurate (like the OS conversion
software) while GPSU and the GPS are using a simpler, faster
conversion?

Yes, this is correct. The Memory Map conversion from WGS84 to pixels
uses a polynomial function with parameters for each map. Both "GPS
Utility" and most GPS receivers use the standard datum and grid
transformation algorithms as published by various cartographic
agencies.

Thanks for confirming that Alan.

However, the OS grid is not truly uniform and there are small (rubber
sheet) distortions.

The Memory Map polynomial conversion method can deal with this problem
on a local map level,

If you want to minimize the difference between the GPSU algorithmic
grid and OSTN02 then please see the Help topic in GPSU on 'Ordnance
Survey Grid'. This describes a method for defining local OS grids
which work over a limited area. If you are using 25K maps then you may
wish to use this technique in GPSU.

But that shouldn't be necessary if you're working with Memory Map though,
should it? I'm using MM to place waypoints which I then export to GPSU for
further editing (waypoint icons etc.) and saving.

If you are transferring your data referenced to WGS84 datum then you
are right - there is no need to worry about the problem. GPSU will
accept the data from MM in WGS84 and transfer the data to GPSs in this
form.

The problem only arizes if you wish to view the data, or compare the
actual coordinates with other data (e.g. Memory Map or real maps).
Because GPS receivers use the uniform grid, the GPS coordinates will
not accurately match the OSTN02 definitive coordinates.

In GPSU you can choose either to match your GPS values or to match the
OSTN02 values. To match the OSTN02 rubber sheet values over a local
area then you will need the method described in the GPSU Help -
however, your coordinates will then not match the values in your GPS
receiver.

The difference in most of England is less than 2 metres, but it gets
worse in north west Scotland. (maximum 15m)

Alan
.

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