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GPS, How-to?

fongu

Active member
I found this location for an old inn and I have the lat and lon for this inn, but I could find no trace of it at the location. I found this site by accident in some maps online and did not even bookmark the location. I did write down the lat and lon numbers, somewhere and I was wondering what reliable GPS unit could I use to punch in these numbers and get right close to this old inn. I just want a basic unit that's easy to understand and doesn't cost a fortune. Can you do this? Either a car GPS or personal GPS to take with you in the wilderness or in town? Any good book references for learning GPS? Any videos or dvd's?
 

fongu

Active member
Thanks Andrew. I intend to check these sites out and look around and see what's out there and exactly which GPS receiver I need.
 

NM5K

New member
I think you would want a hand held for that job. I'm not sure if many of the car units
can go to coordinates. One thing.. I have an older Magellan Meridian and I've never
had too good a luck trying to do that, no matter what type of coordinates I use.
There is always an error.. And I'm not sure why.. :stars:
I've tried lat/lon of the various types, and also UTM. No matter what system I use,
there is an error. I tried this just the other night using an online topo map, and then
checking with the GPS. I always show an error to west, and south. But what is strange
is the map is right on the money.. Go figure... But say if I use the topo map to get
coordinates, and then check the GPS, it will show a couple of blocks to the west,
and a block or two to the south. And each type of system ends up with the same error,
and shows the same exact "off" location. Maybe other units will do better.
As far as the map, it's right on the money though.

Also, even if you get right on the money, that is no guarantee that any of the old
dwelling will be there. One example.. I've got a few acres up in OK on Lake Eufaula.
I ran across an old 1896 topo map of the area and was comparing it to modern topo
maps, and also aerial images. In 1896, Lake Eufaula didn't exist. It was finished in
1964, after being started in 1956. So my old topo maps show the old rivers and
creeks, and also the old roads that were in the area. So instead of the lake being
a few hundred feet north, it was Longtown creek that was a few hundred feet north.
But there were also roads in the area that I had no idea had once existed.
But in comparing the topo maps with the modern aerial images, there is no trace
whatsoever that I can see of the old roads. They were totally grown over by forest.
I was kind of surprised that I could see no traces at all. But looking at the air images,
I could find no traces at all of the old roads. Not even a change in trees or anything.
It's like they never existed. I guess 100+ years will really let things grow over.
Of course, buildings and foundations will stand a better chance of surviving through
the years, but even those are not a sure thing. Depends if they were torn down or
just left to rot away I suppose. A cement foundation will last awhile, but even it
can be grown over and covered with dirt.
Maybe the newer model GPS's will let you go to coordinates better than mine.
It's really strange being as mine seems totally accurate otherwise. IE: the maps
are accurate, and also setting waypoints, etc.. I can go back to a set waypoint
and get very close. I suppose I could calculate the exact error and factor that
in to get to the right place.. ?? :nerd:
 
"I've tried lat/lon of the various types, and also UTM. No matter what system I use,
there is an error. I tried this just the other night using an online topo map, and then
checking with the GPS. I always show an error to west, and south."

When entering co-ordinates to the GPS, make sure of the datum used. Most popular datums in use are NAD27 and WGS84. If your receiver is set to one and you are punching the coordinates for the other, you will get a consistent error just like what you describe.
I don't know which datum you have set now but try changing it to other one and re-entering the co-ordinates. You may find that they are right on.
 

NM5K

New member
Hummm. I think you were right. I checked that, and I seemed to be on a UK datum.. :stars:
I didn't see a NAD27, but it had the WGS84, and I set it to that. I'll do some tests later to see
if that made a difference. Hope so.
 

NM5K

New member
I think that did the trick. I found that I did have both NAD27 and WGS84,
and also a NAD83. I used google earth to pinpoint this house, and also
another online topo map to get a second opinion on the lat/lon.
WGS84 seems to be a good bit more accurate than NAD27. The latter
showing an error to the SE about 100+ feet. It ended up pointing to the
yard next door. But WGS84 seems almost right on the money. Or within
WAAS specs anyway.. I also tried NAD83 just for grins, and it was showing
exactly the same coordinates as WGS84. So I'll leave it on WGS84.
Just hope google earth didn't have gross error... :/ But seems pretty close.
Glad you pointed that out as I wasn't aware that would effect accuracy.
I think when I saw them in the menu, I thought they were map databases
or something.. I had never changed it, but maybe the previous owner did,
or maybe I did by mistake. "I bought it on e-bay about 3-4 years ago."
That the lat/lon never seemed to line up has bugged me for quite a while.
I remember once I was looking at some land, and had coordinates that the
seller had given me. Nothing seemed to line up and I was wondering what
the heck...?? :wacko:
Finally seems to be right now.. Thanks for the tip. :super:
 
Glad it helped.
Datums are basics for any co-ordinates so, just like any other basics, they are often overlooked.
All these basics are covered in "learn the basics" section of our www.GPSNuts.com website.
 

NM5K

New member
Yep, I was looking around the web last night.. Seems you always want to use
the datum that matches the map you are using. IE: some older topo maps use the
older NAD27 datum. Most of the modern maps tend to use the newer datums.
Google earth seemed to use the newer version. Using WGS84/NAD83 it was
pretty close last night. Pretty much the same part of the house, but seemed to
be showing about 5-8 feet northwest of where I'm actually sitting.. Which is within
WAAS specs, and in the house with all the pipes and vents overhead, not the
greatest bird reception. The WAAS bird was kind of intermittent.
And that assumes google earth is right.. It could be slightly off too.
This house is probably where that GPS works the worst.. :/ It's much better
outside, even under heavy trees. Even the car is better I think..
 

fongu

Active member
Think I'll be reading the basics on the website. I read the post about the reciever after I had posted info request and that helped a lot. But, judging from what was said, the GPS will have a Datum setting for older and or modern maps?
 
Yes. Except for automotive receivers meant to be used only with pre-loaded maps, all GPS receivers I ever used had an option to set the datum.
 
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