Misguided Magellan 315

[Rabbit 282]

The short question: My GPSr seems to be "off" by about a half mile. There seems to be a significant difference between my readings and what I've cross-checked on maps and benchmarks. The unit worked absolutely fine when I used it frequently a few years ago, but since I've started operating it again recently the readings are screwy.

 

Magellan GPS 315 (firmware v3.15)

 

The long version:

I took about a two year break from geocaching but just this weekend finally got to get back to it. I dusted off my old Magellan GPS 315 and initialized it for use in new region (I've moved across the country since last I cached). The unit appeared to operate as well as it ever had aside from a battery terminal that had corroded and had to be cleaned. The unit received strong satellite signals and all systems were green (or rather monochrome in the case of this unit ).

 

Once in the wilderness I entered a few cache waypoints and off we went to find them. I found it bizarre that the cache locations were in such remote, difficult to access locations even though their difficulty rating was very low. Of the two we hunted, we found neither cache and had to give up. The clues and descriptions of the cache sites seemed completely irrelevant to the areas I hiked to. I've been unable to locate a cache or two in my day, but this was a bit frustrating.

 

When I returned home I decided to cross check my readings with online mapping systems. Sure enough, my readings/waypoints and the maps' indicators were way off. For example:

My GPSr reading in a nearby parking lot (excellent reception):

45.51082

-122.64593

Versus the coordinates by address results:

45.514863

-122.641213

 

Obviously these don't agree. About .37mi difference. Looking back at the caches on virtual maps, the area of the cache and the area I was looking seem to be consistently offset by the same amount/direction. Calculations for speed and direction seem to be operating just fine, but everything is just about a half mile from where it should be.

 

I tried re-initializing the unit several times (including a few tries after powering down/removing batteries, etc.) using the coordinates provided by the online map, but every time the unit received signal again it would jump back to the wrong coordinates.

 

So... any idea why it would be so inaccurate? And so stubborn to refuse suggested coordinates? Is there some sort of setting that causes this sort of offset? It's an old unit that I should probably replace (Magellan no longer supports it), but I'd like to try to fix it before I go that route.

 

Thanks!

Edited August 28, 2007 by Rabbit 282

[+PDOP's]

Is the Datum set to WGS84 to match the datum used by geocaching.com for geocache coordinates?

[+gunpowder]

I thik that is the same model as my neighbors who bought it online on Ebay. It was way off. I read in the forums how to reset the GPS. We let it set for awhile and it now is in good shape.

[GreatCanadian]

I think the map datum could be the problem as well. Also, it looks like you have the coordinate system set properly but you might want to check that as well. I have a 315 kicking around here for years, and it still gives me accurate readings.

[+geognerd]

The short question: My GPSr seems to be "off" by about a half mile. There seems to be a significant difference between my readings and what I've cross-checked on maps and benchmarks.

The right way to check your accuracy. Use benchmarks that are not listed as being scaled.

 

When I returned home I decided to cross check my readings with online mapping systems. Sure enough, my readings/waypoints and the maps' indicators were way off. For example:

My GPSr reading in a nearby parking lot (excellent reception):

45.51082

-122.64593

Versus the coordinates by address results:

45.514863

-122.641213

The wrong way to check your accuracy. Geocoded addresses are interpolated. Say a street's address range is 100 to 200. The coordinates for house number 150 will be shown at the midpoint of the street. It is highly doubtful that the house is located at the exact midpoint of the street. But what if the address range data is wrong and the house numbers actually only go to 150? Then the example house would be at the end of the street but the erroneous address range would put the house in the middle. Using a geocoder for the US will only get you to the right block on a street.

Edited August 28, 2007 by geognerd

[GreatCanadian]

Anyone else near you with a GPS? Check against theirs to see if the coordinates are close.

[Rabbit 282]

Is the Datum set to WGS84 to match the datum used by geocaching.com for geocache coordinates?

 

Hmm... I think this may be the issue. Hadn't even considered that the datum could have been changed- not sure how that happened. I'll give it a test today and see if this clears things up. Thanks!