NAD27 to WGS84 translation accuracy

[Guest JClishe]

When using your GPS to translate back and forth from NAD27 to WGS84 and vice versa, do you lose any accuracy?

For example, if I enter the coordinates to a Geocache into my eMap while it's in WGS84 mode, then after the waypoint is entered I switch my GPS to NAD27 and go search for the cache, will it be as accurate as if I were to leave it WGS84?

Also, the eMap has several versions of NAD27 listed (Central, Cuba, Greenland, etc). Which version should I be using to properly match the Topozone maps? I'm assuming Central.

 

Thanks

 

Jason

[Guest peter]

quote:

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Originally posted by JClishe:

When using your GPS to translate back and forth from NAD27 to WGS84 and vice versa, do you lose any accuracy?

For example, if I enter the coordinates to a Geocache into my eMap while it's in WGS84 mode, then after the waypoint is entered I switch my GPS to NAD27 and go search for the cache, will it be as accurate as if I were to leave it WGS84?

Also, the eMap has several versions of NAD27 listed (Central, Cuba, Greenland, etc). Which version should I be using to properly match the Topozone maps? I'm assuming Central.

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You shouldn't lose any accuracy by letting the eMap convert. 'Central' would be for Central America. For the 'CONtinental US', you want to use NAD27CONUS.

[Guest fairbank]

Actually, the "CON" in "CONUS" stands for conterminous.

 

Eric

[Guest BigFig]

You'll lose "some" accuracy depending on what conversion algorithm the software or GPS is using. I use NADCON/CorpsCon which - according to the manual - has conversion errors less than one meter for most locations. I've noticed that the topozone conversion discrepancies seem to be a bit larger - I would expect conversions within the GPS to be pretty close. In all cases, the differences appear to be small enough to be ingnored. I think you'll probably get bigger errors converting both coordinate systems and datums - like WGS84 HDDD MM.MMM'

to NAD27 UTM.

[Guest topografix]

Your GPS stores all data internally as WGS84. The fact that it is displaying coordinates to you on the screen in NAD27 doesn't change the internal data format.

 

- Dan Foster

TopoGrafix (http://www.topografix.com)

Software and Waypoints for your Garmin, Magellan, or Lowrance GPS

[Guest BigFig]

Guess I must have missed something here. Yes,

the internal format is WGS84 and that certainly does not change - but you still have to do a conversion based on the internal WGS84 data to display NAD27. The WGS84 data held in the GPS is just as accurate as it was to start with, but the NAD27 coordinates being displayed are still subject to conversion errors regardless of whether the conversion algorithm is based in GPS firmware or software on a computer.

[Guest JuDe]

I've notice some have problem doing convertions of datum.Well I'm going to put my two cents.I have a Mag 315 and that is my converter.I have a screen that allowes me to set a "pri" and "sec" datum.Exemple I have my GPS set at NAD27 UTM as the primary datum and coordinates for use with my compass and my topomaps.For Geocache WGS84 LAT/LONG as the secondary.I will be given a coordinate for Geocache.I move to my "projection" screen,enter the coordinates in lat/long in the secondary that is already set at WGS84,punch that in and the projection screen gives me the coordinates in the datum set in the pry & sec NAD27 and WGS84,also the deg ºM and distance for the compass.I also can keep the both of them together on one screen (NAD27 in UTM & WGS84 in LAT/LONG).This is done on my first NAV screen and done by toggling the L/R button.No calculation for me I let the GPS do the math !! Jude

 

Jude