UTM to lat/lng depends on datum ? (USGS maps)

Dear Experts,

I hope this is a good place to ask this; if anyone can suggest somewhere better, please do!

I have some code that converts between UTM and lat/lng. I've basically copied the "Redfearn's Formula" method as described in the spread*** here: http://www.ga.gov.au/geodesy/datums/calcs.jsp#trans . This seems to work in as much as I get the same answers as various online UTM-to-l/l converters.

However, I'm confused when I look at a USGS map. These maps typically use the NAD27 datum, but have text and marks in the corners describing the offset to NAD83 (which I understand is essentially identical to WGS84). If I have the WGS84 UTM co-ordinates of a landmark and look it up using the map's UTM grid, the position is displaced - but it's not displaced by the amount described by the text and corner marks. It seems to be correct E-W, but it's about 200m too far S.

Thinking about it, I believe that the problem is that the ellipsoid parameters are used in the L/L to UTM conversion. So this conversion is tied to one particular datum (or at least to one particular ellipsoid).

So the whole business of UTM vs lat/lng has suddenly got a lot more complicated for me. Is my understanding correct? Is it true that, given a WGS84 UTM co-ordinate (e.g. a USNG grid reference!), it's not possible to look it up on a USGS map without using some additional information to determine the correct grid displacement?

Thanks for any advice!


Phil.
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