Mapping/GPS mismatch

[Guest BrownMule]

When I plot this particular cache on Mapsource Topo it shows it about 13 feet from the water (on land) on the south bank of the river.

 

BrownMule

[Guest King Pellinore]

There's a cache near here that the pinpoints on the various maps are not where my GPS was pointing when I was visiting the area. I know that Topozone (which is my favorite) uses a different datum and will occasionally place the pinpoint in an incorrect position, but I found this also to be the case on mapquest and the other mappings linked from the cache page. How do I know which is correct? It's a hairy area, with convoluted waterways and roads/trails. Markwell, forgive me if this has been asked before! ;-)

[Guest Mopar]

What cache?

[Guest King Pellinore]

Rahway River Rescue, which I believe is deep in some fescue

[Guest stayfloopy]

quote:

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Originally posted by King Pellinore:

Rahway River Rescue, which I believe is deep in some fescue

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I zoomed in on the Mapquest map for that one and the marker is in the river but close to the south bank. Not accurate but probably still within EPE.

 

It's not in the fescue. It probably doesn't need to be rescued. Maybe someone will join in your quescue...

[Guest stayfloopy]

quote:

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Originally posted by King Pellinore:

Rahway River Rescue, which I believe is deep in some fescue

---

 

I zoomed in on the Mapquest map for that one and the marker is in the river but close to the south bank. Not accurate but probably still within EPE.

 

It's not in the fescue. It probably doesn't need to be rescued. Maybe someone will join in your quescue...

[Guest King Pellinore]

Hmm my GPS was on the next bend in the river, south of the water but the other bank! Drove around several times, but I was pressed for time and moved on. I'm thinking to trust the mappage then. Btw, what's EPE?

[Guest martinp13]

I have been on many hunts where the cache was somewhat close to a lake. Sometimes the internet sites say it's in the water, often my GPS map says it's in the water, but when I find it, the cache is safe and dry, usually 20-50 feet from the water.

 

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> Martin

Magellan 330

Don't have time to program and record your shows while geocaching? Get a TiVo !

[Guest martinp13]

I have been on many hunts where the cache was somewhat close to a lake. Sometimes the internet sites say it's in the water, often my GPS map says it's in the water, but when I find it, the cache is safe and dry, usually 20-50 feet from the water.

 

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

> Martin

Magellan 330

Don't have time to program and record your shows while geocaching? Get a TiVo !

[Guest jfitzpat]

When I plot this on Digital topo data (USGS), I get pretty much middle-of-the-river:

 

http://www.lostoutdoors.com/map.php?map=40.630483,-74.291383 &t=2&z=2&s=1&id=&mark=40.630483,-74.291383,255,0,0,7,*

 

But, when I plot it on NAPP images, it looks like it is on the north bank, near an erosion fork:

 

http://www.lostoutdoors.com/map.php?map=40.630483,-74.291383 &t=1&z=2&s=1&id=&mark=40.630483,-74.291383,255,0,0,7,*

 

When I convert the coordinates to NAD27 using www.jeeep.com, then use Topozone, the location seems to be pretty much exactly where I got using the digital files directly:

 

http://www.topozone.com/map.asp?z=18&n=4497758&e=559893&s=25

 

The problem with mapping sites is that they, generally, use the Tiger line data from the US Census. The margin of error is pretty high, because everything is broken down into straight lines. In this case, the marker looks squarely south of the river, but you can see that the river is drawn as a series of straight lines:

 

 

It should be noted that the maps used by Garmin and Magellan are often based upon this same data, which is why the 'you are here' circle on the maps get so big when you zoom in.

 

Personally, I'd bet on the NAPP image being the closest, but we'd have to ask someone who has found the cache.

 

-jjf

 

[This message has been edited by jfitzpat (edited 21 February 2002).]

 

[This message has been edited by jfitzpat (edited 21 February 2002).]

 

[This message has been edited by jfitzpat (edited 21 February 2002).]

[Guest stayfloopy]

quote:

---

Originally posted by King Pellinore:

Hmm my GPS was on the next bend in the river, south of the water but the other bank! Drove around several times, but I was pressed for time and moved on. I'm thinking to trust the mappage then. Btw, what's EPE?

---

 

EPE stands for estimated position error. Someone will correct me if I'm wrong.

 

By the way, not that my recollection is perfect, but I think I finally found it on the north bank of the river. However, when I was at the cache site, my GPS was still pointing into the river but within EPE. Then again, that location with the twisty river and convoluted streets had me befuddled enough that I arrived at the same wrong bank starting from two different parking locations.

[Guest stayfloopy]

quote:

---

Originally posted by King Pellinore:

Hmm my GPS was on the next bend in the river, south of the water but the other bank! Drove around several times, but I was pressed for time and moved on. I'm thinking to trust the mappage then. Btw, what's EPE?

---

 

EPE stands for estimated position error. Someone will correct me if I'm wrong.

 

By the way, not that my recollection is perfect, but I think I finally found it on the north bank of the river. However, when I was at the cache site, my GPS was still pointing into the river but within EPE. Then again, that location with the twisty river and convoluted streets had me befuddled enough that I arrived at the same wrong bank starting from two different parking locations.