Coordinates

[angelika]

Hi, I just started geocaching and I`ve got a basic question: on what system are the coordinates based on: Is it the UTM ore the WGS-System?

best wishes for christmas, angelika

[+trail hound]

WGS84 ...This should be the default setting on your GPS'r ....MERRY CHRISTMAS and happy hunt'n

Edited December 26, 2004 by trail hound

[+Markwell]

The are based on WGS-84, but you can use whatever system you like. If you automatically load waypoints via your computer and a connector cable, and if your GPS is set to UTM, you don't have to worry (if that's what you like to use).

 

Also, every cache pages main coordinates on the page are also listed in UTM, so you can hand enter them.

 

The only time you would have a problem strictly working in UTM (if that's your preference) would be if it's a multi-stage cache, and you need to get the coordinates at the first stop and hand enter in the coordinates for the second - and repeat the process for the third, fourth, nth stage (or have multiple important coordinates listed on the cache page in the description).

 

You can of course change your GPS's datum display to WGS-84 for entering in the auxilliary coordinates and then change the GPS back to the UTM and the unit will convert, but that seems like a pain.

 

Edit datum/display. Thanks for the correction.

Edited December 25, 2004 by Markwell

[+Poindexter]

And the correct answer is:

 

WGS84 is a "datum", UTM is a "coordinate system". Two entirely different things. To make it simple:

 

There are two sets of coordinates given on each cache page, one in "lat/lon" and the other in "utm", both are in the WGS84 datum. So leave your receiver in it's default datum of WGS84 and everything is fine. You only need to change your datum if you are entering coordinates manually that are of a different datum, or you are plotting onto paper maps of a different datum.

 

Poindexter

[n0wae]

What Poindexter said.

[+AtoZ]

UTM is a coordinate units WGS84 is a Datum type. And typically WGS84 is used but not always, probable 99.999 of the time some people may place a cache using NAS27 and state such but people don't always pay attenting so your be looking 40 meters off where the cache is LOL.

cheers

[+Learned Gerbil]

Also, every cache pages main coordinates on the page are also listed in UTM, so you can hand enter them.

All caches? UK ones carry OS Grid instead of UTM.

[bigben1948]

Whoa, I'm a very novice beginer at this. I've done my share of machine work and a datum in this, never changes, it's always the same exact spot. If I understand you, datums in GPS's change? It may help me to understand the WGS84, NAS27 and other nomenclature if they're acronums, if you'll give me what they stand for? Please go slow, again, I'm new to this sport and trying hard to learn. Coordinates I understand. But, your saying there are different ways of how coordinates are set, like different measurement or method of measuring, ie: metric and fraction/miles and kilometers? Also, if someone could explain the different ways of displaying coodinates, WGS84 and NAS27, why two or more ways, why not standardize this to make it simple for us dummies? ANY HELP I CAN GET IS APPRECIATED, THANKS.

[ozarkray]

If I understand you, datums in GPS's change? It may help me to understand the WGS84, NAS27 and other nomenclature if they're acronums, if you'll give me what they stand for?

 

As I understand it NAS27 stands for North American Survey of 1927 and WGS84 stands for World Grid Survey of 1984. And, yes, they do start in two different spots. And, yes, they did come up with different coordinates for the same spot.

 

Majority of the topo maps published by the USGS have grids based on the NAS27 since that was what was in effect when many of the roads in this country were built and land surveys were made. People who do a lot of work with topo maps use NAS27. GPS coordinates, however, are a world wide effort and most are based on WGS84.

 

Hope that helps,

Ray

[+NickL]

I’ll give this a try. There are several datums that are used to model the shape of the earth. Unlike what most people are taught in school the shape of the earth is not a sphere. As the “body of knowledge” about the geometry and geology of the earth grew new datum were developed and standardized by assorted standardizing bodies. (I didn’t say this was simple). WGS84 is the datum that was used in developing the models for GPS. In addition to defining the center of the earth it also defines, among many other things, the location of the poles and the prime meridians. The difference between the assorted datum is know and can be calculated. What datum you are using is important to your GPSr because it needs to know were the center of the earth is relative to the satellites to compute your location and supply you your location that you can find on a map.

 

Coordinate systems are not as numerous but many do exist to meet the needs of assorted users. The coordinate system is in dependent of the datum but generally uses the datum to define were it starts dividing the earths surface. Also for global coordinates the coordinate system will used the geoid (fancy name for the nonsperical shape of the earth) as defined in the datum when it is gridding the surface.

 

So simply put: the datum defines the center and shape of the earth surface and the coordinate system divides that surface up into a grid.

[+gis_girl]

Maybe I'll pipe in my 2 cents.

 

The North American Datum 1927 is referred to as NAD27. It was developed in 1927 before the geodetic scientists saw earth from space -- it is centered in Meade's Ranch, KS b/c it is the geographic center of the continental US. So, think of that as 0,0 reference point. Now the State Plane coordinate systems for all US states use this datum as a starting point so that distances, calculations, etc. can be made to a common point on the face of the Earth (well in the US) and we can figure out relative positions of things with better accuracy for mapping. However, it's not so good because being positioned on the surface, accuracy can decrease because of mountains and topographical changes.

 

Then came along satellites and spacemen and they developed NAD83, which is more accurate positionally since the geodetic scientists could approximate the real size, shape, mass/density of the Earth -- it's referred to as an "Earth centered datum" because the 0,0 "point" is at the approximated center of the Earth -- better accounting surface elevation and such than NAD27 then. But, they didn't do a bad job back in 1927 since I *think* the average difference between NAD27 and NAD83 is about 200 meters off -- this is why many of those USGS benchmarks are in the wrong position (not the only reason, but often b/c they are based on NAD27).

NAD83 is mostly used for the US Stateplane coordinate systems. WGS84 is another datum as described very well above.

 

The datum is the mathematical model that guesses what the true shape, mass, etc. of the Earth is.

 

A projection is another mathematical model for scaling down the real world (which is technically an Oblate ellipsoid if you ever play Jeopardy!) on to a flat piece of paper (or bits and bytes in a computer map). There are 100's and 100's of projections because when you scale down an area of the Earth to fit on a flat surface you will distort either: shape, area, direction, or distance measurements. No map can preserve ALL of these relative measurements -- only one or maybe two. So no map is EVER 100% accurate. All of these projections are designed depending on what type of use the map is intended, such as measuring true distance, showing true relative area, or navigating where angles of measurement are cruicial. The Mercator projection is good for navigating, any straight line shows true direction but not distance and notice how areas are greatly distorted in the poles (think of that horrible map on your grade school wall -- why everyone thinks Greenland is huge). In simpler terms, think of a projection as wrapping a piece of paper on the earth and the outlines of stuff you want on the map have wet ink.

 

A coordinate system is based on a projection, which uses a datum as the frame of reference for wrapping that piece of paper around the part of the Earth you want to map. Latitude and Longitude is an angular coordinate system that most people are familiar with -- it divides the earth (whatever the shape calculation based on the datum) into degrees of measurement.

 

Since measuring in degrees isn't easy (1 degree of latitude is not the same distance across at the poles as it is at the Equator) and doing that kind of math, many coordinate systems have been developed to take that projection of the Earth (based on a datum) and divide it into "arbitrary" zones that everyone can use as a relative frame of reference -- and they generally use FEET or METERS for measuring (much easier than Radians!).

 

For example, the UTM coordinate system divides the Earth into 60 zones both North and South of the Equator. The UTM system is good when mapping in ONE zone. It is based on the Transverse Mercator projection and often uses NAD83 or WGS84 for the datum and METERS for the measurement scale.

 

The GIS data collected by local county and government agencies in the United States, generally use their own Stateplane Coordinate system. For instance, California has 6 zones in the California Stateplane system -- for example the system might be refered to as CA_ZoneIV_NAD83 based on FEET. Each state has it's own coordinate space because depending upon its latitude, its area, its shape, etc.. one of those 100's of projections out there was chosen as the best one to preserve those 4 most important measurements: area, distance, direction, and shape. The distortions of those 4 are much less when mapping a smaller area -- and thus you have happier map users.

Edited December 30, 2004 by gis_girl

[ozarkray]

The distortions of those 4 are much less when mapping a smaller area -- and thus you have happier map users.

 

Right! Happier map users, but not happier GPSr users.

Ray