using the right coordinates

[ghengis]

when looking at the geocache info page, which coordinates should one use for locating the cache? all of the utm coordinates are given in wgs84 format but all of my topo maps are un the nad27 format. will the conversion in jeeep. be correct, to use with my nad27 maps?

[Aksor+Raskol]

I would set up my GPSr to use WGS84 format, because that's what you'll be using to do 98% of the hunt. If you still need your topo maps, convert the NAD27 coordinates to WGS84.

[Scout03]

quote:

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Originally posted by Aksor+Raskol:

I would set up my GPSr to use WGS84 format, because that's what you'll be using to do 98% of the hunt. If you still need your topo maps, convert the NAD27 coordinates to WGS84.

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Could you enter the coordinates into the GPS in WGS84, then set the GPS to NAD27 in order to use the map? Wouldnt the GPS do the conversion for you?

 

I am new to this also . . . hence the question.

 

Scout03

[Swagger]

quote:

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Originally posted by Scout03:

Could you enter the coordinates into the GPS in WGS84, then set the GPS to NAD27 in order to use the map? Wouldnt the GPS do the conversion for you?

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Can't speak for other brands/models, but my eTrex Legend does this.

 

--

There were plants and birds and rocks and things,

there was sand and hills and rain...

[4x4van]

Yes, the Magellan does this also. In fact, the Magellan gives you two sets of coordinates at once; your primary, which I have set to DD MM.MMM in WGS84, and your secondary, which I have set for UTM in NAD27. Pushing the right (or left) arrow while on the position screen gives you the secondary coordinates.

 

We don't stop playing because we grow old...we grow old because we stop playing!

[jimburns]

quote:

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Originally posted by ghengis:

all of the utm coordinates are given in wgs84 format but all of my topo maps are un the nad27 format. will the conversion in jeeep. be correct, to use with my nad27 maps?

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> If you still need your topo maps, convert

> the NAD27 coordinates to WGS84.

 

Or update your maps. My USGS Topos are all NAD83 and feature only transformation ticks to NAD27. They are 8 yrs old.

 

Also, take a look at,

 

Geo Convert

 

for a tool that does format and coordinate conversions all at once. In addition to some other nice features. I'm thinking of adding a configuration then to support at least NAD27, NAD83 and WGS84.

 

I'd like to get a feel for whether it's worth supporting these others for the geocaching community, NAD27 at least. For NAD83 and WGS84 they're still sometimes referred to as using the GRS80 ellipsoid as if they were exactly the same.

 

NAD83 and WGS84 are so close with identical radii and differing by only .0000014162 in flattening. In contrast to NAD27 with uses the Clarke 1866 Ellipsoid of significantly different radius and flattening. It may be worth it just to support NAD27.

 

HTH and any feedback appreciated,

 

/jim/pearland/tx

[Kerry.]

The differnce between NAd83 and WGS84 is of little significance for practical applications and certainly as far as geocaching is concerned it's simply not an issue. In pure ellipsoidal compuational terms the difference is around 0.0001 metres so even for most surveying purposes is of academic interest only.

 

However one should look at NAD83 and WGS84 as being independent of each other as they are basically based on different points (mainly), different observation data etc. This different base coordinate information then gives rise to any difference in physical coordinates much more so than any differences in ellipsoids.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[jimburns]

quote:

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Originally posted by Kerry:

However one should look at NAD83 and WGS84 as being independent of each other as they are basically based on different points (mainly), different observation data etc. This different base coordinate information then gives rise to any difference in physical coordinates much more so than any differences in ellipsoids.

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And... So? Not sure how you're suggesting this is meaningful for geocaching but when talking about the GPS system the ellipsoid, which we commonly refer to by datum, is the most meaningful piece to the puzzle. Yes, the datums may involve different sets of terrestrial points but as we're not out there surveying ourselves the difference in physical points is irrelevant. The GPS system maps right to the ellipsoid it's based upon, not to a database of known points. The known points that make up the datum don't play a part in a GPSr determining a position or in converting from one set of points to the other. They are merely that, a set of base reference points. Moreover the GPS system is causing a refinement of the base points so that they MORE CLOSELY fit the underlying ellipsoid.

 

Still, not sure of your point, my point was only that NAD83 and WGS84 are certainly much closer than NAD27. When moving from GPS data to older NAD27 based maps I can see this difference being enough to warrant good translation.

 

Regards,

 

/jim/pearland/tx

 

-- Jim Burns

Pearland, TX

[+Alan2]

While conversion in most GPS's will get you the "right" spot on NAD27 quad maps, for accurate translation, check NADCON which is NOAA's 150,000+ sampling of actual point in the US to make the conversion more accurate.

 

Alan

[Kerry.]

Jim, put simply the difference between NAD83 and WGS84 is really not an issue so supporting BOTH (NAD83 & WGS84 if that’s what you meant above) as far as transformations/conversion from/to NAD27 for geocaching is really not warranted.

 

And for sure there’s more than enough difference between WGS84 and NAD27 to require appropriate translation but NAD27 being the confused hybrid it is, straight mathematical conversions aren’t really suitable anyway and all this has to do with the underlying data sets and adjustments over many years.

 

Also that “meaningful piece to the puzzle”, “spheroid” is a more appropriate definition than “ellipsoid”, according to the OED definition anyway. In any case the spheroid on which GPS is based had to be determined first and the IERS have been refining a global datum ever since. WGS84 was modified and changes made to GPS in 1994 to be coincident with ITR92. Any spheroid is determined based on/from physical points, just that’s it’s much more complex to do this for global datum purposes than a simple local definition.

 

And yes, what most commonly call a DATUM is the geographical coordinates, namely latitude, Longitude referenced to the surface of the spheroid. So if we’re talking about datum’s then the WGS84 datum does include geographical coordinate sets but if we’re referring to the WGS84 spheroid, hic ellipsoid (which is in fact based on the GRS80 spheroid except for f) then that is different and this is where technically the NAD83 & WGS84 datum’s could be thought of as different even if they were based on the same spheroids (which they aren’t).

 

Users work in datum’s not spheroids and there’s several examples in the world where different datum’s use an identical spheroid yet the coordinates are many metres different. So with a common spheroid any conversions/translations between these “datums” are based on the underlying data sets in conjunction with the spheroid/s but not simply the spheroid only.

 

Any good conversion/translation between GPS (WGS84) and NAD27 should be done using NTv2 distortion models as NAD27 data sets are not what one would call homogenous.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Team Svenofthenorth]

Kind of a new question. Been at this a while, but came upon a multi cache that used two different formats for the coordinates. Had to plug the numbers in to a code to get the second set. The first set was using degress-minutes-seconds and the second set was using the decimal coordinates. If you didn't pay attention this put you miles off. No mention of different formats in the cache either. Is this normal? Not the only one having difficulty finding this cache.