GPS Map Doesn't Match

[Curious The George]

I have a Garmin Etrex Venture HC GPS. I use MapSource topo maps on it.

There is a place where an interstate highway's lanes cross each other. The north bound side crosses a bridge over the south bound side. This makes a very accurate "X" on the map.

When driving across that point in both directions with the GPS turned on the track is missing that X on the map by about 520 feet.

How do I get the map moved to match the actual location that the GPS says is the actual crossing point? I could see it being off by a few feet but a tenth of a mile is kind of too much.

[golddigger14s]

Double check your map DATUM's. Both the GPS and mapping software should be set for the same Datum, usually "WGS-84".

[WishfulThinker]

Go read up on the different Surveys .. especially for older maps .... as accuarcy improved, maps shifted origins and earth curvature values ... this is detailed in some of the sailing/boating publications, of all places, because there, accuracy really does count, when you are trying to navigate a channel or coastal obstruction.

"This is not a joke, read the identification block on the map to determine which reference datum was used."
Some maps of coastal areas may be from sea charts of ancient origin, and may/or-may-not have been corrected to CURRENT standards. 1/10 mile or 500 Ft error is not uncommon on translating old map positions. Google Earth is good reference as to what is really the correct position.

[Sky Pilot]

I can't begin to tell you how many folks come by my neck of the woods looking for Norsford Rd., a very handy shortcut to town shown on their GPS.
Here's the kicker: Norsford Rd., was a ford across the river that the Indians (and everyone after them, until the 1930's) used centuries before white folks came here!
Yet Google Earth and every GPS system I've seen will direct a person to cross the river there!
I would think they could do better than that, but oh well.
Regards, SkyPilot

[TerryC]

The biggest problem with maps not matching is budget cuts. The USGS is no longer THE SOURCE for US topos. It is outsourced to 4 different places now. And these no longer send as many people to the field to check accuracy. 4 sources, less field checks... you can expect the problem to get worse! TTC

[maui]

terry, thanks much for the info. good to know.
take care and be carefull out there. ron

[TerryC]

terry, thanks much for the info. good to know.
take care and be carefull out there. ron

Hey Maui,
It is not info I wanted to acknowledge, let alone pass on. I wanted to clarify my computer mapping software by buying the four topos that converge where hwy 49 crosses the Melones reservoir near here. The software is correct AS IT SEES THE TOPO PRODUCT! The four topos do not match. Also, I can see 4 glaring mistakes within 200 yards of the house! I have had a passion for maps since I was a kid. Todays lack of accuracy is driving me bonkers! I'll stop here before getting profane. Ugh! (take care) TTC

[maui]

hi terry, thanks much for the info. and like you said it is not good when you get differant
readings from the differant maping systems. i thought they would all be pretty close but
that's not the case as you have pointed out.
take care and be carefully out there. ron