Topo Zone Coordinates

[Guest streamer]

Has anyone else noticed that Topo Zone map coordinates don't seem to match the posted GPS coordinates for a cache location. Why?

[Guest jeremy]

Topozone uses the outdated Nad27 datum, which can be up to 500 feet from the actual wgs84 coordinates we use. I'm working on doing a conversion so the Topozone maps match up to the Geocaching.com maps -

 

The big issue, however, is if people use the Topozone maps to enter in their coordinates (which would be wrong in WGS84). Hopefully everyone uses the ones listed on Geocaching.com.

 

Jeremy

[Guest robanna]

I found a cache today using only a TopoZone map and a compass! I guess I just got lucky that the cache was right where TopoZone said it would be.

[Guest fairbank]

This seems to be the biggest problem that new GPS users are having. They don't understand datums. It's not that TopoZone is "using" the outdated NAD27 datum. That's the datum that almost all of the USGS topo maps are projected in. It's perfectly fine to use these maps, just remember to always set your GPS to use the same datum as the map you are using. Coordinates are useless for accurate navigation without also knowing the datum for those coordinates. Your GPS will do the conversion for you. If you have a lat/lon in WGS84 and you want to use these maps which are in NAD27 then first set your GPS to WGS84, enter the waypoint coordinates, and then set your GPS for NAD27 and the waypoint coordinates will be converted and then you can use the maps. I have a help page for those interested in downloading and using these topo maps yourself. You'll get a much better printout than you can get at TopoZone and you can do a lot more with these maps.

http://www.geocities.com/fairbank56

 

Eric

[Guest edmcnierney]

Hello from the TopoZone! I've been trying to help out GPS users who work with our maps, and it sounds like things are off to a good start here already.

 

The NAD27 datum isn't "outdated", because there are still over 50,000 USGS printed topo maps being sold that have the NAD27 grid and coordinates on them. That's why we chose NAD27 as a default for the TopoZone. For applications such as geocaching, there's absolutely no benefit to using NAD27 vs. WGS84 or just about any other datum; none are better or worse than the others, you just need to be sure your GPS and maps agree.

 

We're planning on adding datum conversion options to TopoZone as soon as we get around to it. Of course, then folks will start to complain because their coordinates won't agree with the grid lines they see on our maps !

 

If anyone needs help using TopoZone maps, just have them send email to info@topozone.com and we'll get right on it.

 

- Ed

 

Ed McNierney

Chief Mapmaker

TopoZone.com

[Guest jeremy]

I think we're both right, and both wrong about datums and their outdatedness (if that's a word )

 

Anyway, to GPS folks, the standard is WGS84. So for us, we consider NAD27 as being outdated.

 

For the topo maps on Topozone, you guys use NAD27, which is pretty much the standard for older maps. So in the way of map reading, NAD27 is still considered very current.

 

If you have a good math algorithm (not a program) that I could use, I'd love to have a copy and put this issue to rest. Hopefully people will use our coordinates so they don't get confused though.

 

contact@geocaching.com

 

Jeremy

[Guest jeremy]

I think we're both right, and both wrong about datums and their outdatedness (if that's a word )

 

Anyway, to GPS folks, the standard is WGS84. So for us, we consider NAD27 as being outdated.

 

For the topo maps on Topozone, you guys use NAD27, which is pretty much the standard for older maps. So in the way of map reading, NAD27 is still considered very current.

 

If you have a good math algorithm (not a program) that I could use, I'd love to have a copy and put this issue to rest. Hopefully people will use our coordinates so they don't get confused though.

 

contact@geocaching.com

 

Jeremy

[Guest edmcnierney]

Well, I guess I consider myself one of the "GPS folk", too . The primary reason your GPS units are set to WGS84 is that it's an international standard so it's never really "wrong". But it's also rarely "right" for a printed map user.

 

The chief reason most GPS users choose WGS84 is because that's what their GPS did when they took it out of the box, and they don't know what a datum is so they'd never feel interested in changing it!

 

However, on a worldwide basis you will find the vast majority of topographic maps do NOT use the WGS84 datum. They use a variety of local datums, or they simply use older datums because the maps were last updated prior to the existence of the WGS84 datum. So WGS84 is likely to (generally) disagree with just about everything. However, it's the datum that everyone is willing to disagree with equally .

 

The National Geodetic Survey has a number of geodetic software tools available at http://www.ngs.noaa.gov/PC_PROD/pc_prod.shtml - including NADCON, which can be used to compute datum shifts. It's available with source code. There's an online version at http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html that will let users interactively convert coordinates.

[Guest edmcnierney]

One more thought - be careful of your desire to have GPS users use "our coordinates so they don't get confused". Forget about TopoZone for a minute. A GPS user who sets a GPS to WGS84 and reads a coordinate from it, and then plots that coordinate on an NAD27 USGS topographic map (i.e. most of them) will locate themselves in the WRONG PLACE! I call that getting confused.

 

I'd love it if all the USGS topographic maps used WGS84. But they don't, and we need to help GPS users correctly plot their positions on real maps, not the ones we wish they had.

[Guest Cape Cod Cache]

I use a GPS48, it was meant for use in a marine environment. NOAA charts use WGS84. I have a nice old chart on my wall that used NAD27. It also has Lightships on it. USELESS!

I don't know how often topographical maps are updated, But I get new charts every couple years or so. There are a slew of warnings printed on every chart, and guide books state that you better have the newest charts available. Being 70 feet off a cache site might be a drag, but sinking is worse. Alot of towns are using GIS to keep track of data for use by police, fire, water etc. I checked my town and they use WGS84.

Just a thought

[Guest Mike_Teague]

The answer is, the USGS 7.5" quads that are so highly praised are NEVER updated... Some were created at least 50 years ago.. They have at the best, (in populated areas) been updated slightly in the last 10 years by aerial photographs... The majority are over 20 years old... Maybe topozone doens't disclose this, I don't know..

 

Nevertheless, I dont know many people who rely on an outdated USGS quad for guidance during a geocache hunt.. I have found very limited use for them myself... For one thing, the scale is not what can be considered useful when looking for such a small object.. Sure, you can pull out yer little UTM grid overlay (or make one) and plot the position of a cache to the meter, (theorectically), but... Do you think they are that accurate?

 

You _might_ be able to get a _general_ idea of what kind of terrain to expect, but I wouldn't bet on that either.. There is so much that can happen in even a tenth of a mile as far as terrain, that wont show up on a topo map... I.e. a 30 foot deep gouged-out creekbed (wont show up on the map with a 40 foot contour interval!) --- I've had this happen! Climbing down a 30 foot cliff, and up the other side IN THE DARK can at least make your day interesting

 

I think more importantly, look at what the map you have says, if you choose to use one... They are good, but I'd no sooner rely solely on a USGS 7.5" quad than a GPS by itself..

 

Anyhow, map & compass dudes can go ahead, try a geocache at NIGHT sans GPS!! bwaahahahahaha! Convert the lat/lon WGS84 cache coords to UTM on NAD27, go out with yer compass and map, and good luck! (try a cache without clues as well!)

 

heheee

[Guest Mike_Teague]

The answer is, the USGS 7.5" quads that are so highly praised are NEVER updated... Some were created at least 50 years ago.. They have at the best, (in populated areas) been updated slightly in the last 10 years by aerial photographs... The majority are over 20 years old... Maybe topozone doens't disclose this, I don't know..

 

Nevertheless, I dont know many people who rely on an outdated USGS quad for guidance during a geocache hunt.. I have found very limited use for them myself... For one thing, the scale is not what can be considered useful when looking for such a small object.. Sure, you can pull out yer little UTM grid overlay (or make one) and plot the position of a cache to the meter, (theorectically), but... Do you think they are that accurate?

 

You _might_ be able to get a _general_ idea of what kind of terrain to expect, but I wouldn't bet on that either.. There is so much that can happen in even a tenth of a mile as far as terrain, that wont show up on a topo map... I.e. a 30 foot deep gouged-out creekbed (wont show up on the map with a 40 foot contour interval!) --- I've had this happen! Climbing down a 30 foot cliff, and up the other side IN THE DARK can at least make your day interesting

 

I think more importantly, look at what the map you have says, if you choose to use one... They are good, but I'd no sooner rely solely on a USGS 7.5" quad than a GPS by itself..

 

Anyhow, map & compass dudes can go ahead, try a geocache at NIGHT sans GPS!! bwaahahahahaha! Convert the lat/lon WGS84 cache coords to UTM on NAD27, go out with yer compass and map, and good luck! (try a cache without clues as well!)

 

heheee

[Guest edmcnierney]

We "disclose" the date of every map we use - just click on the "Quad Info" link right next to the quad name above the map.

 

But your argument is that USGS quads aren't good maps to use. I'm neither lobbying for or against that. USGS quads do go a long time between updates, but your dates are exaggerated and I don't think you've really looked at what the USGS has on a nationwide basis.

 

But I don't want to debate that point. My point is a very simple one, and it applies to topo maps, nautical charts, and everthing else. Here goes:

 

If we want users to be able to use a GPS to locate points on a map with < 100 meter accuracy, we need to educate them about what a map datum is. There is no "right" map datum in a vacuum; the right datum to use is the one your map uses.

[Guest Cape Cod Cache]

Just a quick sidebar:

I was looking at both the USGS and NOAA/USCG/FAA sites relating to cartography. USGS maps are from the Dept of Interior, NOAA/USCG/FAA charts and aviation maps are from the Dept of Commerce. In both of their mission statements, the interaction between Government agencies is stated as a future goal. Perhaps an inter-departmental agency will exist. Both also are investigating interaction with state, local and private concerns. Inclusion of GIS with satellite tech from everybody ( use a CRAY ! ) could make a fantastic map. With new technologies ever emerging, I bet in 10 years it will be a whole new picture in cartography. Just think of the advances your computer has made in the past 10-20 years. Hmmmm....

[Guest n1niq]

I have a question that I hope is on topic here. On my Garmin II, I have the option to select datum type (i.e. WGS84) and then I also have the option to select how the coordinates are displayed numerically. Can someone shed light on this or direct me to some information?

[Guest fairbank]

First, do a search on this site on these topics, and then if you have more questions, ask away. The search link is at the top of the page.

 

Eric

[Guest peter]

rrectly datum WGS84 and found that I hadn't needed to climb the hill afterall since the cache was in a gully at the bottom.

 

Are you still working on converting the coordinates? Otherwise it would be nice to at least have a warning message next to the Topozone hyperlink.