WGS84 vs NAD27 on the Cache Pages

[Guest Hamster]

Hey folks,

I remember some discussion about this before but I don't remember if there was a resolution.

 

When clicking on the "Topozone" maps (which are in NAD27) it seems to me that the link on the GeoCaching site is transmitting the decimal format coordinates, but in WGS84 format. I clicked on the "convert to NAD27" link, and saw on one of the new caches that this was the case. I then punched in the NAD27 coordinates on the topozone URL, and sure enough the little pointer doohickey moved.

 

I remember there was some talk that the site code may be changed so that on those URLS the reference datum would be converted... whats the status? Am I misunderstanding? If its not already been done, I could write a .dll ASP class could be used by Jeremy to do the conversion.

[Guest jfitzpat]

You are correct, the link on the page passes WGS-84 coordinates, TopoZone expects NAD-27. The difference can be substantial. For example, when I plug a GPS waypoint from my balcony into our free Aerial/Topo generator, the dot it dead on my balcony in the image, and on the right spot on the topo.

 

If I plug the WGS-84 coordinates into TopoZone, the dot is on the roof of a house two streets over. Interestingly, if I convert the data to NAD-27 and plug it in, the marker is still slightly off - about 2-20 meters. It also moves when I select print or Zoom. I think that this is because TopoZone is drawing the marker client side with my Browser.

 

I just embed the marker(s) in the topo and aerial images before delivery, so I don't have the same issues.

 

I actually have ASP script code to do the conversion, which anyone who wants it is welcome to. I also have the same thing in PHP, but it is slightly less accurate (at least with the PHP math functions under Apache). I also have an Active Server Component that could do the transformation. It's overkill, but it would be easy to strip down.

 

-jjf

[Guest Hamster]

quote:

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Originally posted by jfitzpat:

substantial. For example, when I plug a GPS waypoint from my balcony into our free Aerial/Topo generator, the dot it dead on my balcony in the image, and on the right spot on the topo.-jjf

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Could you post the URL to your free util?

[Guest jfitzpat]

http://www.lostoutdoors.com

 

The Map Maker is for building USGS images (topo and aerial). You can download the markers you place diretly to your GPS if you are running IE and don't mind accepting an ActiveX control. (I've been plugging away at a Netscape plug-in (pun intended), but I can't seem to get a solid chunk of time to finish it).

 

The Waypoint Exchange uses the same ActiveX control to email waypoints to others.

 

The Adventure Database just went online (a single entry, the hike I took last weekend with my son - but many to follow now that the data entry scripts are done).

 

Enjoy.

-jjf

[Guest Hamster]

a web-programmer by hobby or career. Do you do contract work? I run a small firm that does alot of web-based database work, so I'm always interested in this kind of thing.

[Guest Markwell]

Used www.lostoutdoors.com for a Geodash hunt last weekend. I LOVE this utility. Please keep it online forever and ever!

[Guest Markwell]

Used www.lostoutdoors.com for a Geodash hunt last weekend. I LOVE this utility. Please keep it online forever and ever!

[Guest Hamster]

I for one would love to see a link to the lostoutdoors maps from the geocaching page. There in one place you have both types of maps we need. And the pointer could actually be in the correct place!!!

 

Very very nice resource.

[Guest jfitzpat]

For Hamster:

 

My day job is programming, but not normally web stuff. I do a lot of device drivers and signal processing stuff. I have embedded a HTTP server in some turnkey devices, and done backend stuff for a lab monitoring website, so I've dabbled. We do have two engineers at eTree who are real web whiz kids.

 

One of them is a regular climbing partner, so after I get all the backend pieces working, I'll brow beat him into giving the site a real facelift.

 

Oh, for the record, (you'll see the pun in a minute), eTree is not a .com company. Our name predates the IBM eBiz campaign. Our name actually comes from the Elvis Tree (a ficus tree with a tiny Elvis Action Figure in it). We almost named ourselves "Elvis Tree", but then worried about getting sued.

 

For Markwell:

 

Well, "forever and ever!" is a really long time. If it is any consolation, I still take the occassional call about a multimedia control system I did for amusement parks back in '88 and laser graphics projection system I last worked on in '86...

 

-jjf

[Guest arffer]

Jeremy once responded that the reason he passes WGS-84 data to topozone in the link is that he is still looking for a good WGS-84 to NAD-27 conversion routine, and was looking for suggestions.

 

In my own experiance, I once ran a set of co-ordinates through 7 different conversions; some were online sites and some were PC programs. Each one gave a different set of results, some substantially off.

 

------------------

Team CacheCows of Wisconsin

[Guest Alan2]

quote:

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Originally posted by arffer:

Jeremy once responded that the reason he passes WGS-84 data to topozone in the link is that he is still looking for a good WGS-84 to NAD-27 conversion routine, and was looking for suggestions.

 

In my own experiance, I once ran a set of co-ordinates through 7 different conversions; some were online sites and some were PC programs. Each one gave a different set of results, some substantially off.

 

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I noticed that my Garmin MapSource converts to different coordinates than the "Jeep" conversion routine in the Geocaching site. Am I nuts or do others get the similar results?

Also, I left my Vista in my car so I don't know if the GPSR converts the same as Mapsource. Gee I hope so.

 

Alan

[Guest jfitzpat]

Mapping Projections in general and NAD-27 in particular are not trivial problems. I could give some reading references if someone really wants to dig in.

 

In a nutshell, projection isn't a straight forward mathmatical formula. There are several projection libaries floating around (Proj.4 and GCTPC are two popular ones). There have been a fair number of versions of each, and different math libraries can give different results on different platforms...

 

Small wonder different vendors give different answers.

 

Even the same vendor might not even always give the same results. Example, Garmin converts the a USGS coordinate database to WGS-84 for its maps (everyone else does as well). If you change the map datum on an eTrex and enter in coordinates directly from the USGS text files, then switch datum back, you'll get a slightly different value than on the Garmin map!

 

So, you either run a reasonable converter, and get pretty close on TopoZone, or just feed the WGS-84 coordinates to me...

 

-jjf

 

[This message has been edited by jfitzpat (edited 13 December 2001).]

[Guest jfitzpat]

Mapping Projections in general and NAD-27 in particular are not trivial problems. I could give some reading references if someone really wants to dig in.

 

In a nutshell, projection isn't a straight forward mathmatical formula. There are several projection libaries floating around (Proj.4 and GCTPC are two popular ones). There have been a fair number of versions of each, and different math libraries can give different results on different platforms...

 

Small wonder different vendors give different answers.

 

Even the same vendor might not even always give the same results. Example, Garmin converts the a USGS coordinate database to WGS-84 for its maps (everyone else does as well). If you change the map datum on an eTrex and enter in coordinates directly from the USGS text files, then switch datum back, you'll get a slightly different value than on the Garmin map!

 

So, you either run a reasonable converter, and get pretty close on TopoZone, or just feed the WGS-84 coordinates to me...

 

-jjf

 

[This message has been edited by jfitzpat (edited 13 December 2001).]

[Guest arffer]

quote:

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Originally posted by Alan2:

I noticed that my Garmin MapSource converts to different coordinates than the "Jeep" conversion routine in the Geocaching site. Am I nuts or do others get the similar results?

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That's exactly what I was talking about... I used 7 different conversions and got seven different answers. The conversion that the GPSR makes internally is one of the seven, so I can't vouch for it being any more accurate than the other 6.

[Guest Alan2]

States, more than 200 meters in Alaska, Puerto Rico and the Virgin Island and in excess of 400 meters in Hawaii. Consequently, the shift between the various datums are not uniform across the United States and there is no single value that can be applied to latitudes or to longitudes based on old datums to convert them to NAD 83." NADCON was developed in order to facilitate conversion between the datums,. (from NAD 23 to NAD 83 and vice versa)

 

To give an example, my Vista and Mapsource converts NAD 23 UTM 18 T 566000 4568000 to NAD 83 18 T 566034 4568212. If you compare to the actual 7.5 quad map from the USGS, the first set matches positions the waypoint fairly acurately on the actual 7.5 map which is in NAD 23. However, the 7.5 map says the offset for NAD 83 should be 6 meters south and 35 meters north. Garmin is pretty close on the latitude (34 vs. 35 metrers) but is way off on the longitude - 212 meters vs. 6. Interestingly the map itself no longer states the correct difference. The actual NADCOM calculation is 10.398 meter north latitude not 6. NADCOM calculated longitude is pretty close to the map 34.185 vs. the map's 35.

 

Bottom line is the shift is not uniform and would reqire 150,000 points in a date base across the country to calculate it more accurately. I imagine that no one uses that database and is picking some sort of a common adjustment number used in thei calculations across the board.

 

If you want to get an accurate conversion utility see the USGS NADCON site http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html

 

Alan2

[Guest jfitzpat]

FWIW, NADCON isn't always that accurate in all cases either. There are some rounding problems at the edge of some grid files, sometimes giving meters of inaccuracy.

 

Also, the results may not always match a printed map because the map itself can be slightly in error.

 

Still, the results are about as good as you are going to get.

 

-jjf

 

P.S. I finished the Netscape plug-in. My cohort Dave has promised to squeeze in some testing this weekend so we can post it soon.

 

In the mean time, I stuck up part of the Geographical Name Information System database in the Map Maker. You should now be able to search for place/feature names in the 5 western states (about 1/4 million waypoints). I'll post the rest the states 5-10 at a time over the next few days.

[Guest Hamster]

Wow, I just was messin' with one of the caches I've been to thats in a very urban area http://www.geocaching.com/seek/cache_details.asp?ID=473

 

I hit the Terraserver and topozone links on the page and the images were Waaaayoff. Whereas when I put the same WGS84 coords into LostOutdoors' mapmaker, It was right on!!

 

I wish we could get a link to LostOutdoors site from each cache page