Datums

[HWyatt]

Datums make my head hurt.

But this is what I think I understand.

 

Maps come in different scales, just like architectural drawings. These scales are called datums.

 

When I am figuring out utm's on my old topo map and it has a datum of NAD27ConUS, I better have my gps set on that datum.

 

When I am entering way points from the geocache site (which I do by hand as I am a proud Mac Computer user) I should reset my gps to WGS84...as that appears to be the datum most caches are listed under.

 

But what if I forget to switch my datum when entering waypoints & I keep the gps in NAD27 ConUS....I enter the way points at home and once on the hunt I realize my mistake and switch over to WGS84. Will the gps automatically redo the way points or must I reenter them?

I think I better reenter them.... Yes...no....????

 

Thanks for your help.

 

HWyatt

 

edit ps: when I am out of utm what should my gps be set to:

hddd.ddddd°

or

hddd°mm.mmm'

or

hddd°mm'ss.s"

Edited December 19, 2004 by HWyatt

[+Blaidd-Drwg]

OK, first Datum has nothing to do with scale. Think of Datum as a starting point for all measurements in Northing, Easting and Altitude (X,Y,Z). A NAD 27 location will be off from a WGS 84 location by about 250 meters. If you have your GPSr set to NAD 27 and enter the location as seen on a cache page, then you will be entering a wrong location. Changing your GPSr after entering the location will just compound the location. So YES, once you realize you've had your GPS set to the wrong datum when you entered the location, you should re-enter the location with the correct datum set. The Datum must match the location. As for running out of UTM, I don't know what you mean. There is a UTM location for the entire earth, exluding the poles. But the correct format for Lat/Long is ddd mm.mmm and is how it is displayed on cache pages. All locations on cache pages are in WGS84 Datum. I hope this answers your questions.

[+Sputnik 57]

More on datum lore.

[+NightPilot]

The format doesn't matter, as long as you know what it is and enter the coordinates in the correct format. The format display is just a display. But you have to put the right numbers in. If you send the waypoints from your PC, it doesn't matter at all what your GPS display is set to show, the waypoints will be entered correctly. The GPS stores the waypoint as a number, and manipulates that number as needed to display it the way you want.

[n0wae]

The Geocaching site uses WGS84 datum so use that datum when you type in the cache way points. You will find the cache just fine even if using NAD27 maps. The shift between WGS84 and NAD27 is only about 120 feet or so. If you want to plot a road on a map so that it matches *exactly* to the map like surveyors do then you might be concerned about datums. For caching leave it at WGS84. I use laptop navigation with 7.5 Topo maps with NAD27 datum and the moving icon (my Jeep) matches close enough to map features using WGS84 datum. I use to switch back and forth until I forgot to switch back and posted one of my caches in the wrong datum. A 120 foot error when your driving is no big deal but finding a cache with that same error is indeed a problem. Set your equipment to WGS84 and leave it there.

[+Harry Dolphin]

Yes. You do have it wrong. Plural of Datum is Data.

[+Rakusan]

Sorry, Harry, but cartographers and geodesists DO use the word "datums".

 

"Data" are raw material for generating information.

 

"Datums" is the multiple of "datum" when we're referring to the "geodetic datum" - a mathematical model of the exact, or best-approximated, size and shape of the Earth.

[+Harry Dolphin]

Harry goes off topic. (And this is unusual why?)

There are specific area with which I am unfamiliar. Each area of expertise has its own lingo.

Unfortunately, modern English defers to the least common denominator. This is called the 'dumbing of America.'

Forgive Harry for his error. He is a stick in the mud, and thinks that baseball is played in stadia; that these are the Geocaching Fora. No less an authority than IMDG now prints 'indexes' ather than 'indices'. And Harry refuses to accept that the word 'agendas' exists!

I stand corrected. (Actually, I'm sitting corrected.)

[+Poindexter]

The shift between WGS84 and NAD27 is only about 120 feet or so

 

The shift varies depending on location AND coordinate system used. If your using lat/lon, the difference of 120 feet is typical (96' where I live in Maryland) but is as high as over 300' in portions of California. If your using UTM, it is typically MUCH greater. In fact, at my location, it's about 700'. So, if your using paper topo's and UTM to plot your position on the map, make sure you've selected the proper datum when entering positions manually in your gps, and then select the proper datum to display coordinates to use with your map.

 

Poindexter

[HWyatt]

The shift between WGS84 and NAD27 is only about 120 feet or so

 

The shift varies depending on location AND coordinate system used. If your using lat/lon, the difference of 120 feet is typical (96' where I live in Maryland) but is as high as over 300' in portions of California. If your using UTM, it is typically MUCH greater. In fact, at my location, it's about 700'. So, if your using paper topo's and UTM to plot your position on the map, make sure you've selected the proper datum when entering positions manually in your gps, and then select the proper datum to display coordinates to use with your map.

 

Poindexter

Got it. Use the correct datums & write the coords on paper to stick in a pocket. So when I realize I have entered the right coords in the wrong datum I can correctly make the change.

 

My gps needs a datum toggle switch.

 

Thanks, as always you all are an education.

 

HWyatt