position format and what does format and datum mean to me?

[+weylan]

I am using the Nuvi 500 and am trying to learn more about it.

 

I was using one format for a puzzle cache but was wonding.

 

If I change the format or DATUM, does this effect the accuracy? If so in general if I am geocaching, in Californa what format and datum should I be using?

 

And what is datum?

 

The format I understand I think, which is pretty straightforward.

Edited January 15, 2009 by weylan

[+Prime Suspect]

I am using the Nuvi 500 and am trying to learn more about it.

 

I was using one format for a puzzle cache but was wonding.

 

If I change the format or DATUM, does this effect the accuracy? If so in general if I am geocaching, in Californa what format and datum should I be using?

 

And what is datum?

 

The format I understand I think, which is pretty straightforward.

While there are local datums, what you should be using is WGS-84. It is a world-wide datum (a mathematical model of the shape of the earth), and is what all the coordinates on this site use (certain puzzles excluded).

[+PDOP's]

If I change the format or DATUM, does this effect the accuracy? If so in general if I am geocaching, in Californa what format and datum should I be using?

 

Geocache coordinates are posted decimal minute format( DD mm.mmm). Coordinates are in the WGS84 datum (that's noted at the bottom of every cache page).

[+twolpert]

If I change the format or DATUM, does this effect the accuracy? If so in general if I am geocaching, in Californa what format and datum should I be using?

 

And what is datum?

If you are geocaching, you should be using the WGS-84 datum. That is the datum in which geocaching.com publishes the coordinates. It doesn't matter where you are.

 

A datum is a relationship between coordinates and a physical point on the earth. Although it's an oversimplification, think of a datum as providing the "origin" for the coordinate system. Two coordinate systems (datums) with different origins will have different coordinates for the same spot. Use a football field as an example. Suppose I am standing on the five yard line at one end of the field. If I count from that end, it's the 5. If I count from the other end, it's the 95. So I have two different coordinates for the same spot. Same thing with datums.

 

As long as you remember one important thing, you don't need to be concerned with datums while geocaching. Always set your unit to the datum in which the coordinates are given before you enter those coordinates. Practically speaking, this means set your unit to WGS-84 and leave it there. You generally will not have a reason to change it unless you use your unit in conjunction with a paper map which uses a different datum (for example, USGS topo maps which typically use NAD27 CONUS).

 

Choice of datum does not affect unit accuracy. However (and it's a big however), entering coordinates given in one datum with the unit set to a different datum will put you in the wrong spot when you navigate to those coordinates. Hence the warning to make sure the datum is correct before entering coordinates.

 

Choice of format does not affect accuracy except in so far as the different formats have slightly different accuracy limitations based on the number of decimal places they use (and the significance of those decimal digits). Practically speaking, you can ignore these differences.

[+weylan]

This helps thanks for the good explanation.

 

 

I guess this could explain why some people might also have some accuracy issues when geocaching too?

 

 

Say you set your datum to NAD27 CONUS, because you don't know any better.

 

Then you access or download a geocache to the GPS and try to navigate to it.

 

 

While it might be close, the navigation to the geocache might not be right. And this could explain why some people complain that the GPSr do not take them directly to the coordinates.

 

Do I have this understanding correct, or did I miss something? Is one more accurate then the other. I understand we are stuck with the WGS-84 for geocaching and it would be a pain to change, but I was just curious.

 

 

If you are geocaching, you should be using the WGS-84 datum. That is the datum in which geocaching.com publishes the coordinates. It doesn't matter where you are.

 

A datum is a relationship between coordinates and a physical point on the earth. Although it's an oversimplification, think of a datum as providing the "origin" for the coordinate system. Two coordinate systems (datums) with different origins will have different coordinates for the same spot. Use a football field as an example. Suppose I am standing on the five yard line at one end of the field. If I count from that end, it's the 5. If I count from the other end, it's the 95. So I have two different coordinates for the same spot. Same thing with datums.

 

As long as you remember one important thing, you don't need to be concerned with datums while geocaching. Always set your unit to the datum in which the coordinates are given before you enter those coordinates. Practically speaking, this means set your unit to WGS-84 and leave it there. You generally will not have a reason to change it unless you use your unit in conjunction with a paper map which uses a different datum (for example, USGS topo maps which typically use NAD27 CONUS).

 

Choice of datum does not affect unit accuracy. However (and it's a big however), entering coordinates given in one datum with the unit set to a different datum will put you in the wrong spot when you navigate to those coordinates. Hence the warning to make sure the datum is correct before entering coordinates.

 

Choice of format does not affect accuracy except in so far as the different formats have slightly different accuracy limitations based on the number of decimal places they use (and the significance of those decimal digits). Practically speaking, you can ignore these differences.

[+PDOP's]

Is one more accurate then the other. I understand we are stuck with the WGS-84 for geocaching and it would be a pain to change, but I was just curious.

 

It's not so much it being more accurate but a different model of the earths shape. If you are given coordinates in NAD27 and have your GPS set to NAD27 you should be able to navigate to that point as accurately as with any other datum. The key factor being that the datum of the points is the same as the datum set in your GPS.

[rlmartin58]

If I change the format or DATUM, does this effect the accuracy? If so in general if I am geocaching, in Californa what format and datum should I be using?

 

Geocache coordinates are posted decimal minute format( DD mm.mmm). Coordinates are in the WGS84 datum (that's noted at the bottom of every cache page).

 

I have downloaded some data from a gov web site in the following format. How can I convert the coordinates to somthing usable

 

0774511W, 400305N, Berger Trail, 1169291 Trail Franklin PA Fannettsburg 585 1919 - 2-Aug-79

0773202W, 394715N, Biesecker Gap, 1169473 Populated Place Franklin PA Waynesboro 265 869 - 2-Aug-79

0773158W, 394544N, Biesecker Run, 1169474 Stream Franklin PA Waynesboro 205 673 - 2-Aug-79

0774502W, 400330N, Big Gap, 1169511 Gap Franklin PA Fannettsburg 446 1463 - 2-Aug-79

0775612W, 395701N, Big Mountain, 1169539 Summit Franklin PA McConnellsburg 740 2428 - 2-Aug-79

0775602W, 394357N, Blair Valley, 1169796 Basin Franklin PA Clear Spring 283 928 - 2-Aug-79

0773910W, 400844N, Blue Mountain Tunnel, 1169890 Tunnel Franklin PA Doylesburg 535 1755 - 2-Aug-79

0772817W, 394327N, Blue Ridge Summit, 1169894 Populated Place Franklin PA Blue Ridge Summit 410 1345

0773254W, 394706N, Blue Rock School, 1169895 School Franklin PA Waynesboro 236 774 - 2-Aug-79

[+twolpert]

Say you set your datum to NAD27 CONUS, because you don't know any better.

 

Then you access or download a geocache to the GPS and try to navigate to it.

 

While it might be close, the navigation to the geocache might not be right. And this could explain why some people complain that the GPSr do not take them directly to the coordinates.

 

Do I have this understanding correct, or did I miss something? Is one more accurate then the other. I understand we are stuck with the WGS-84 for geocaching and it would be a pain to change, but I was just curious.

In my experience (in a limited local area, and it does vary by location), the difference between NAD27 and WGS-84 is on the order of a small number of 10s of meters, or less. So yes, in theory, this kind of thing could account for not arriving exactly at the cache.

 

However, the much more likely explanation is accumulated error. Chances are, the posted cache coordinates are off by at least a little bit. The position your GPSr reports as you approach the cache is also off by a little bit. If they are off in opposite directions, a couple of "little bits" add up to enough to cause a problem. An error of 30 or 40 feet is not all that unusual. That's why a lot of cachers just put the GPSr away and start using their eyeballs when they get close.

 

"Stuck with" has kind of a negative connotation The choice of datum for any given use is kind of arbitrary. Geocaching uses WGS-84 because it's the latest datum and spheroid that's an accepted international standard. And because that's what your GPSr uses internally.

[+twolpert]

I have downloaded some data from a gov web site in the following format. How can I convert the coordinates to somthing usable

 

0774511W, 400305N, Berger Trail, 1169291 Trail Franklin PA Fannettsburg 585 1919 - 2-Aug-79

0773202W, 394715N, Biesecker Gap, 1169473 Populated Place Franklin PA Waynesboro 265 869 - 2-Aug-79

You can if you know what format they are in and what datum they use. I'm no expert, but this one does not ring a bell. They look a little like UTM, but if so, the zone is missing. (The zone would be a two-digit number and a single letter, such as 18T. It would normally appear in front of the other two sets of numbers.) Which means it's probably something else I don't recognize. Hopefully, somebody else out there will recognize it.

 

Might help if you posted the URL for the site where you got it, if the site is open to the public.

 

Edited to add: Also, UTM coordinates would normally be expressed as an easting and a northing. Offsets are measured from the southwest corner of the grid, and increase as you go east and north. So the first number of the coordinate pair is normally followed by an E if any indication is given at all.

Edited January 15, 2009 by twolpert

[+markandsandy]

 

I have downloaded some data from a gov web site in the following format. How can I convert the coordinates to somthing usable

 

0774511W, 400305N, Berger Trail, 1169291 Trail Franklin PA Fannettsburg 585 1919 - 2-Aug-79

It's in degrees, minutes, seconds format.

 

Just split it like I show below.

 

0774511W, 400305N

077 45 11W, 40 03 05N

077 degrees 74 minutes 11 seconds West, 40 degrees 03 minutes 05 seconds North

 

Hopefully the website gives info on what datum is used.

[+JBnW]

If I change the format or DATUM, does this effect the accuracy? If so in general if I am geocaching, in Californa what format and datum should I be using?

 

Geocache coordinates are posted decimal minute format( DD mm.mmm). Coordinates are in the WGS84 datum (that's noted at the bottom of every cache page).

 

I have downloaded some data from a gov web site in the following format. How can I convert the coordinates to somthing usable

 

0774511W, 400305N, Berger Trail, 1169291 Trail Franklin PA Fannettsburg 585 1919 - 2-Aug-79

 

These look like Degree, Minute, Second coords to me. Of course, I could be wrong. One way to translate them to another format is using http://www.gpsvisualizer.com/gpsbabel/

 

I manually changed the cited coords to N40 03 05 W077 45 11, entered those into Google Earth, and came to some spot near Fannettsburg, PA, although I have no idea if this is the trailhead for the Berger Trail. As was mentioned before, outside of geocaching, there are legitimate reasons to use something other than the WGS-84 datum. Our State Park agency also produces waypoints in a similar format, but uses NAD 83 and/or NAD 27 (and/or WGS-84). Hopefully, the website from whence these came also mentions the datum the coords are based upon. Hope this helps!

[+twolpert]

As was mentioned before, outside of geocaching, there are legitimate reasons to use something other than the WGS-84 datum. Our State Park agency also produces waypoints in a similar format, but uses NAD 83 and/or NAD 27 (and/or WGS-84). Hopefully, the website from whence these came also mentions the datum the coords are based upon. Hope this helps!

NAD83 and WGS-84 are close enough that they can be used interchangeably for recreational use with handheld receivers. Not so with NAD27.

[+coggins]

I have downloaded some data from a gov web site in the following format. How can I convert the coordinates to somthing usable

 

0774511W, 400305N, Berger Trail, 1169291 Trail Franklin PA Fannettsburg 585 1919 - 2-Aug-79

 

Berger Trail is a trail in Franklin County, Pennsylvania.

 

Degrees Minutes Seconds:

Latitude: 400305N

Longitude: 0774512W

 

Decimal Degrees:

Latitude: 40.05139

Longitude: -77.75333

 

http://www.lat-long.com/ShowDetail-2594-Pe...rger_Trail.html

[rlmartin58]

Thanks everybody for the help I am going to try working with them as suggested I havnt been able to find my way back to the web site When I do I will verify what it is I am working with

 

 

 

As was mentioned before, outside of geocaching, there are legitimate reasons to use something other than the WGS-84 datum. Our State Park agency also produces waypoints in a similar format, but uses NAD 83 and/or NAD 27 (and/or WGS-84). Hopefully, the website from whence these came also mentions the datum the coords are based upon. Hope this helps!

NAD83 and WGS-84 are close enough that they can be used interchangeably for recreational use with handheld receivers. Not so with NAD27.