Looking for explanation for datum and conversions


Good day

Can anyone point me to any resources they found useful in explaining datums ? And/or a conversion utility between them ?

I have several years of data from the mid-90's that reference NAD27 as my hardcopy maps at the time were based on NAD27 and I setup the GPS (a Garmin 45XL) the same. These days I use WGS84 on the new units.

I'm looking to understand what effect the different datum references will have on my archive of waypoints. I'm looking to map some trails with the old and new data combined.

thank you


Depends on Location

In my limited experience with changing datums, here in the northeastern US, I have seen offsets between NAD27 and WGS84 (NAD83) to be around 600' North-South and 60' East-West. This however will vary greatly with location.

I've found some useful information on the subject from a post at Geonet.com :


The National Geodetic Survey website offers a conversion formula:


I've found it to be a bit awkward to use effectively in some cases. You can substitute WGS84 for the NAD83 coordinates since the offset between the two datums is less than 3' in most of North America.

Wikipedia also offers some basic information on the subject:


thank you -

Thank you - I'll check out those resources



The FCC has tools to convert the datums as well. They use coordinates for the location of transmitter antennas and some were originally posted in NAD and have to be converted to WGS.

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If you have a Windows computer, there is a free conversion utility called Fizzycalc that is used a lot by geocachers to do various datum and coordinate conversions, distance calculations, projections, etc. Just Google Fizzycalc and you will find it.

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