Datum

[+wxfamily]

is datum (WGS84) correct for southeast TN?

 

Mine was set to Tristian 68' and doing a search I keep seing the WSG84 one pop up.

 

Also, how do I get the base map, the yellow/brown one back into a iQue 3600? I deleted it.

[+PDOP's]

is datum (WGS84) correct for southeast TN?

 

Yes All waypoint data on geocaching.com uses this datum.

[+Quanmeg]

is datum (WGS84) correct for southeast TN?

 

Mine was set to Tristian 68' and doing a search I keep seeing the WSG84 one pop up.

 

 

I to seem to be having quite a few problems getting the datum right and it appears to be a multi fold problem for me.

 

First - During the transfer of waypoints from my maping software to my GPS a minor adjustment in the coordinate is made. This is often due to a mismatch between the datum in my maping software and the datum I have set in the GPS. When this happens the GPS corrects the coordinate making it a little off...or is it actually correct? in anycase I have to check and recheck the coordinates.

 

Second- It appears that hiders may be using a different datum than I am using when finding. Yesterday I went to 2 cache locations within 1 mile of each other in Kentucky. At the first one I used the canada datum and my GPS zeroed to within inches of the cache location. Other caches found that have been hidden by this hider have also zeroed using the canadian datum. Using the same datum while searching for a cache hidden by someone different 1 mile away my GPS was zeroing 120 feet from the actual cache location. So for me swithching datums trying to figure out the hider is all part of the find. If all else fails try the UTM

 

Third- The datum I use at home in Texas doesn't always seem to work very good in other locations. Seems to me you just have to try switching things up.

 

Forth - your map may not accuratly dipict your true location. one manufacturer has this to say

 

The quality of signals from the GPS satellites is poor causing reduced positioning accuracy. Such a loss of signal quality may happen for the following reasons:

 

1) The GPS antenna is in an unsuitable location.

 

2) Obstacles are blocking signals from the satellites.

 

3) The position of satellites relative to your GPS is bad.

 

4) Signals from the GPS satellites have been modified to reduce accuracy.

(GPS satellites are operated by the US Department of Defense, and the

US Government reserves the right to distort positioning data for military reasons. This may lead to greater positioning errors.)

 

If a cellular phone is used near the GPS antenna, GPS reception may be lost temporarily.

 

Do not cover the GPS antenna, because this may block the reception of GPS signals.

 

Check the GPS signal reception and the position of the GPS antenna if necessary, or continue moving doing the drunken bumblebee walk until reception improves. Keep the antenna clear.

 

Posting the datum of hiders GPS encripted in the clue section could be helpfull for all.

 

IN anycase using WGS84 produces the most accureate results but it may not put you on point with the cache if the hiders coordinates were established using the tristen 68 datumall you can do in my opinion is be aware in the knowledge this can be a factor in your hunts and be resourceful.

[+wxfamily]

wow, that last reply lost me at hello.

[Alphawolf]

Datums...A major source of confusion among GPS users.

 

There are lots and lots of datums you can use that are built into your GPS. A datum is nothing more than a "reference" that maps are built against. Think of it this way...The datum that map makers use to make a map makes the map accurate for that part of the world, at the expense of other parts of the world.

 

The datum you choose to use depends on two things: First, if you are using a paper map in conjunction with a GPS, the datum in the GPS must match the datum the map is based on. The map will tell you what datum it is based on.

Second, if you are importing coordinates from any other source for inputting into a GPS (say, like from Geocaching.com, or a book or magazine or a friend) then again, your datum must match the source exactly.

 

If a geocaher looking for a geocache uses any datum besides WGS-84, then he will likely miss the mark. WGS-84 is the "default" datum for geocaching. Likewise, if a person hiding a cache is using anything besides WGS-84, then no one will ever find their cache.

 

Lastly, if you are standing in a spot and wish to mark a waypoint with your GPS, it doesn't matter one little bit what datum you are using, your GPS will remember that spot, and get you back to it, no matter what datum was used to mark it.

[Alphawolf]

By the way...If you input a set of coordinates into your GPS, then change your datum after the input, your GPS WILL change those coordinates for you. Automatically. This is normal.

[Alphawolf]

Other caches found that have been hidden by this hider have also zeroed using the canadian datum. Using the same datum while searching for a cache hidden by someone different 1 mile away my GPS was zeroing 120 feet from the actual cache location. So for me switching datums trying to figure out the hider is all part of the find. If all else fails try the UTM

 

No...Trying to figure out what datum the hider used is not all "part of the find". There are well over 100 datums he could have chosen. If he used anything but WGS-84, then he was in error, and no one is ever likely to find a single one of his caches, unless somehow, they find out what datum he erroneously set his GPS to. Someone need to get hold of him and get him speaking the same geocaching language the rest of the geocaching world is using.

[Alphawolf]

If all else fails try the UTM

 

UTM isn't a datum. UTM is a "position format", or think of it this way...just another way to describe a point on Earth. The "official" position format for geocaching is degrees, minutes, and thousandths of a minute (hddd mm.mmm).

You can use any format you want to show your coordinates, and they all mean the same exact spot on the planet. Just different names for them. But to keep things uniform in the geocaching world, we all should be using hddd mm.mmm as a standard. Just so we are all speaking the same language.

 

Again, your GPS doesn't care one bit which format it is set to. It will dutifully guide you to the same spot exactly, no matter which format you currently have it set to.

[stvanme]

If all else fails try the UTM

 

UTM isn't a datum. UTM is a "position format", or think of it this way...just another way to describe a point on Earth. The "official" position format for geocaching is degrees, minutes, and thousandths of a minute (hddd mm.mmm).

You can use any format you want to show your coordinates, and they all mean the same exact spot on the planet. Just different names for them. But to keep things uniform in the geocaching world, we all should be using hddd mm.mmm as a standard. Just so we are all speaking the same language.

 

Again, your GPS doesn't care one bit which format it is set to. It will dutifully guide you to the same spot exactly, no matter which format you currently have it set to.

 

UTM isn't a datum nor is a "position format", it is a coordinate system that can be associated with any datum just like most coordinate systems. It stands for Universal Transverse Mercator.

[Neo_Geo]

Funny... I don't see "Coordinate System" listed anywhere on my GPSr's menu. But if I want to change from Lat/Long to UTM, I can indeed find the setting under "Position Format".

[+Renegade Knight]

Funny... I don't see "Coordinate System" listed anywhere on my GPSr's menu. But if I want to change from Lat/Long to UTM, I can indeed find the setting under "Position Format".

 

That's what your GPS calls it. It's still a coordinate system.

[Neo_Geo]

My point is that it could be called a "Position Format" too (much like a road can also be called a street) - especially when the dominant GPSr manufacturer wants to call it "Position Format".

[+blindleader]

First - During the transfer of waypoints from my maping software to my GPS a minor adjustment in the coordinate is made. This is often due to a mismatch between the datum in my maping software and the datum I have set in the GPS.

Unlikely. WGS84 is the standard for transfer of waypoint data between GPS devices and anything else, and it doesn't mattter what datum is set in either device. You don't mention what your mapping software is, but if it makes a difference what datum you're using (again unlikely), then that software has a serious design flaw.

 

Second- It appears that hiders may be using a different datum than I am using when finding. Yesterday I went to 2 cache locations within 1 mile of each other in Kentucky. At the first one I used the canada datum and my GPS zeroed to within inches of the cache location. Other caches found that have been hidden by this hider have also zeroed using the canadian datum.

I'm not sure what you mean by "Canada Datum" is, but NAD27-Canada differs from WGS84 by about sixty feet in Kentucky. Close enough that I wouldn't make any assertions about what datum the hider was using, unless it was in some urban area where you could get better coordinates using high resolution satellite photographs to improve on GPS fixes.

 

Third- The datum I use at home in Texas doesn't always seem to work very good in other locations. Seems to me you just have to try switching things up.

Contrary to what you might feel in your heart of hearts, there's nothing special about Texas. WGS84 is the one that works worldwide, even in Lubbock. All geocache coordinates are referenced to WGS84. Perhaps your expectations are unrealistic. Sometimes the GPS will report your position within a couple of feet of your true location, and sometimes the error can be well over fifty feet. Furthermore, it isn't always easy to tell one from the other.

 

Posting the datum of hiders GPS encripted in the clue section could be helpfull for all.

At the risk of beating a dead horse. It's already done, not encrypted, and posted at the bottom of the cache pages beneath the oldest log.

 

Don't worry, as you gain more experience, learn the limitations of the system, develope skill in searching, operating the GPS system, and interpreting the results, you won't be sidetracked into worries about datums and maps and such.

 

And just to make sure that horse never makes it to the starting gate:

WGS84 WGS84 WGS84 WGS84 WGS84 WGS84 WGS84 WGS84 WGS84 WGS84 WGS84 WGS84

 

 

edit: PS to comment on this log. :)

Don't ever ever ever do that. Only the cache owner can determine if a cache is missing for sure, and replace it. Neither you nor any of the others who DNF'd this cache really know if you were in the right place.

Edited May 1, 2006 by blindleader

[planewood]

Funny... I don't see "Coordinate System" listed anywhere on my GPSr's menu. But if I want to change from Lat/Long to UTM, I can indeed find the setting under "Position Format".

 

That's what your GPS calls it. It's still a coordinate system.

 

The Universal Transverse Mercator Projection is just one of dozens of ways to project a spherical surface onto a planar (flat) surface. The UTM analogy is like stuffing the earth into a giant tube that touches at the equator, slitting the tube along a constant line of longitude, and then flattening out the tube till it is flat.

 

There are numerical constants used in the formulas when converting lat/lon's on the earths surface to UTM coordinates. Those constants are only good for 6° E/W. Or, 3° on either side of a zones central meridian. The height of the N/S zones vary but are 8° for most populated land areas. When you go to a different area then new constants are necessary. The circle around the equator is divided up into 60 UTM E/W zones, and the semi-circle around the poles is divided into about 20 N/S zones. Each zone has a different set of numerical constants. Converting lat/lons in one zone using numerical constants from another zone will yield incorrect UTM coordinates.

 

Using the UTM constants and equations is most commonly called the 'UTM Projection System'. i.e. - a way to project the earth onto a flat surface like a paper map.

 

http://www.uwgb.edu/DutchS/FieldMethods/UTMSystem.htm

[+Klemmer]

wow, that last reply lost me at hello.

 

wxfamily: Simple answer - just set your GPS at WGS84 datum and forget it.

EXCEPT: if you are trying to get to a spot shown on a PAPER map (it didn't sound like it).

[Alphawolf]

That's what your GPS calls it. It's still a coordinate system.

 

Well, in the interest of not boring common folk to death with endless details about "coordinate systems", I thought it was best to probably call it what their GPS calls it...A "position format". After all, it is a GPS in their hands that they will be using...Not a text book about projections...

[Grasscatcher]

Most (if not all) GPSrs "Think" internally in WGS84. .....but can "Talk" (display) to you in whatever datum you have chosen by your settings on the unit.

 

It's like a person that can speak several languages. (Ever wonder which language they "think" in?) They can talk to me in one language and to you in another ,etc

 

What IS IMPORTANT is that, any data that you input into your GPSR MUST be entered in the same format as you received it.

If you get it as WGS84, set your unit on WGS 84 BEFORE entering the data then enter it.

If NAD 27, change your unit THEN enter.

If NAD 83, change your unit THEN enter.

 

Then your unit will have the correct information internally and will display the correct information to you, no matter which "display" format (datum) you choose.

 

As mentioned above, if you are plotting points from coordinates on your GPSr, or getting coordinates from a paper map to put into your GPSr. Then your GPSr should be set to the same Datum as the paper map(before data transfer either way).

 

Hope this helps.

[Suscrofa]

When you are proficient in one language, you tend to think in it too ! It saves on the "processing" time plus some expressions can't be transalted word for word !

BUT when it comes to computation, you always do it in your mother tongue !

 

Now af for GPS internal processing, I am curious how the "default" is.

Decimal degree for position ? What about distances, is it metric as for virtually all computer controlled machines tools and computaiotn or some other stuff ?

 

AS I plan to get a Garmin 60CX soon, how many decimals does it show when it goes from meters to Km ? I have seen a GPS that above 1600m was displaying 1.7 Km, thus loosing some digits. This is annoying as you have to walk a 100m to notice a difference.

It would make sense to display as much digits as can be like 1.789Km, 18.09Km if you limit the display to 4 digits.

[dave and jaime]

while the default for gps units is generally wgs84 and decimal degreee, the gps doesnt do any type of translation unless you actually change either the coordinate display format or the datum, and then the 'translation' is only for stored data not the displayed data. the datums are simply a mathematically description of the earths surface projected onto a flat sheet of paper, once the gps gathers info, that is timing and location/elevation, from the sats it simply inputs the data into the correct formulas and outputs a position.

 

why wgs84 is the default with decimal degree ill never understand. most mapping is based on either nad27 or nad83 while degrees are pretty useless to convert positions to distance. professionally ive never seen any type of degree format used, actually impossible with survey instruments, nor have i ever encountered wgs84, even though its essentially nad83, as being the datum of choice on any construction project.

Edited May 2, 2006 by dave and jaime

[planewood]

while the default for gps units is generally wgs84 and decimal degreee, the gps doesnt do any type of translation unless you actually change either the coordinate display format or the datum, and then the 'translation' is only for stored data not the displayed data. the datums are simply a mathematically description of the earths surface projected onto a flat sheet of paper, once the gps gathers info, that is timing and location/elevation, from the sats it simply inputs the data into the correct formulas and outputs a position.

 

why wgs84 is the default with decimal degree ill never understand. most mapping is based on either nad27 or nad83 while degrees are pretty useless to convert positions to distance. professionally ive never seen any type of degree format used, actually impossible with survey instruments, nor have i ever encountered wgs84, even though its essentially nad83, as being the datum of choice on any construction project.

 

WGS84 was adopted as the international standard replacing the USA derived NAD83. The difference is something about a slight change in the shape of the earth and accounts for something like 1" in 1000 miles.

 

Don't understand your statement about decimal degrees? The program I use most often will let me specify dd.dddddd, dd.mm.mmm, or dd.mm.ss.sss. One other will sense the type of format and adjust accordingly.

 

Oh I see, you said distance. Yep, lot easier to convert UTM coords to distance.

[+GPSlug]

WGS84 was adopted as the international standard replacing the USA derived NAD83. The difference is something about a slight change in the shape of the earth and accounts for something like 1" in 1000 miles.

It was close to NAD83 when it started out (for some reason, the Defense Mapping Agency wanted to use something that was also defined outside of North America), but with continental drift and more refined models, it's periodically updated to some flavor of ITRF. It's about 1.4 meters different for me now.

 

Don't understand your statement about decimal degrees? The program I use most often will let me specify dd.dddddd, dd.mm.mmm, or dd.mm.ss.sss. One other will sense the type of format and adjust accordingly.

 

Oh I see, you said distance. Yep, lot easier to convert UTM coords to distance.

Minutes are handy for nautical miles. Internal to the GPSr though, decimal degrees are much easier for computations. Actually, the raw positioning is done in Earth-Centered, Earth-Fixed cartesian coordinates in meters. Then it's rotated to the surface of the Earth for navigation.

[dave and jaime]

Minutes are handy for nautical miles. Internal to the GPSr though, decimal degrees are much easier for computations. Actually, the raw positioning is done in Earth-Centered, Earth-Fixed cartesian coordinates in meters. Then it's rotated to the surface of the Earth for navigation.

 

calculations inside a gps are done using the satellite as the center with the users(ie gps) position on the surface of a sphere of radius equal to the distance to the sat as computed from the time recieved. as more sats are locked 'onto', there become several spheres and your position would ideally be the center of the overlap region/volume.

 

in actual fact there is no global standard as it regards to datums. datums only have a value when coordinates are applied to maps, to my knowledge maps are rarely produced in wgs84 and local datums are generally used.

[+GPSlug]

calculations inside a gps are done using the satellite as the center with the users(ie gps) position on the surface of a sphere of radius equal to the distance to the sat as computed from the time recieved. as more sats are locked 'onto', there become several spheres and your position would ideally be the center of the overlap region/volume.

That's how it's described, and that's how you might do it if you were doing it by hand, but that's not how the calculations are done. The satellite positions are calculated into ECEF and an estimated receiver position is used to throw it all into a least squares adjustment or Kalman filter (iterated if you don't have a good estimate yet). The center of the Earth is a much more convenient coordinate origin that moving it satellite to satellite.

 

in actual fact there is no global standard as it regards to datums. datums only have a value when coordinates are applied to maps, to my knowledge maps are rarely produced in wgs84 and local datums are generally used.

It depends on your user group. For consumers and commercial users, regional datums make sense for maps. The main use for wgs84 here, though, is to have a common system from where you can translate to whatever you might need locally.

 

For space applications and large-scale geophysics, you want to stay in a global model like wgs84 or ITRF. The US military also prefers to have all their maps in a consistent, global reference frame.

 

Reminds me of a quote: "The great thing about standards (or datums) is that there are so many to choose from."