----------------------------Original message----------------------------
Courtney Schumacher wrote:
>
> ----------------------------Original message--------------------------> I'm curious about DMA map accuracy in the central/western, tropical
> Pacific.  A meteorological research project that my group is involved
> with is underway in the Marshall Islands. We have 17 rain gauges
> between 7-9.5 N and 166-169 E. A GPS receiver was recently sent out to
> get exact lat/lons of the gauges, however, the GPS unit at times
> disagreed with DMA nautical maps of the area (map numbers: 81030,
> 81715, 81723, 81557, 81737). The largest disagreement was almost 20 km
> but in general the discrepancies were on the order of a km. What is
> the DMA standard for island locations on their nautical maps? Are the
> locations accurate to within a km? I understand that there could have
> been errors in the GPS readings but I'm trying to bound the possible
> error of the DMA maps. Any information or references would be
> appreciated!
>
 
I don't have anything to do with DMA/NIMA hydro charts, but I did have a
lot to do with many other series of charts. I spent 1983 perfecting our
orthophotos and eliminating any possible error. Orthophotos were and are
still used as a control base for the manual extraction of planimetric
detail for our charts and maps. The usual error in our control orthos
before I tamed them was rather large. Afterwards, the error was always
less than 100m and usually less than 50m. So, throwing in the
compilation error, the resulting error for a manually produced map would
be nowhere near the error you experienced; that could only be a blunder
or a mixup in islands. The compilation date of the charts you used would
also be important since I would only expect the accuracy I cited to be
on maps compiled after 1983.
 
 
Kurt Dikkers
NIMA, but not speaking for them.