Gps Accuracy Question

[+Prof_Fate]

Hi all - we are new to geochaching and have found 3 so far (1 real, 2 virtual). Sorta found 1 more virtual (right place, but cannot figure out what specifically we ar elooking for) and hence my question...

 

I have a Garmin eTrex Legend and get 'accuracy' reading of 27 to 60+ feet (it varies as i watch it). When looking for a cache i see i am close, +/- the above 20-60 feet. Then i see it says go East and i start that way and then it says go west...sometimes moving my 'destination' by 100 feet or more....but the built in compass can tell i moved, and what direction with only 1 step (3' of movement)....

 

So is this variation in location a gov't induced inaccuracy, my unit or just normal?

 

Also, using WGS84 and the traditional degree/minute/second notation, how far in feet is each division (i.e. 40d 42.400 is how far from 40d 40.410?) Are lat and long the same in feet for a given measurement in seconds?

 

Thanks in advance,

 

chris and jill

[+AuntieWeasel]

Pretty normal. I don't pay much mind to the stated accuracy, but experience tells me 25' is about my average error rate. So, take your 25' and add the 25' of the hider, and you can expect to be 50' off on average.

 

As you may imagine, this is a very rich and much discussed issue. I'm sure someone will be along shortly with some handy pointers to useful resources -- but in the meantime, you can just scroll down the page and find other threads covering accuracy, heading, bearing, and avoiding the drunken bee dance.

[+tirediron]

Don't get too wrapped up in the accuracy displayed by your GPSr. The EPE (Estimated Positional Error) is basically a 'guesstimate' that the unit makes based on the number of satellites and their relative position.

 

Once you get close to your destination, it's perfectly normal for the GPS to start moving erratically, how much will again depend on how many satellites you have etc. The best way -IMHO is to aproach on a direct path until the pointer starts to jump. Get a visiual reference of the direction you want to travel (or take a compass bearing) and then repeat the process from a couple of different angles, noting the bearing and the distance to go to the destination. In theory, the 'cache is at the intersection of those points

 

Remeber, your GPS is off by as much as 50 - 60' and so possibly was the hiders. The GPS only gets you to the aerea of the 'cache. After that, it's up to you! The difficulty rating on the cache page will give you a rough idea of how tough this cache will be to find.

 

Look for things that look just a little out of place.. a too neat pile of rocks, some moss that doesn't quite match the area, a hollow tree, basically, where you would put a container if you had to hide in the area.

 

As to your question about lat/long... One degree of latitude is 60 miles. I minute is therefor one mile. 1/10 of a degree is 608' (this is based on nautical miles) and so on... Longitude varies depending on the position. Latitude is parrallel, longitude is not.

 

Welcome and good luck!

 

Edit: Durn quick-fingered mustelids!!....

Edited September 28, 2004 by tirediron

[virgo91967]

First question: the inaccuracies you are geting are not uncommon and are not a result of government induced inaccuracy. Inaccuracies in location can have many sources including satellite orientation and signal path multiplexing ( sat signals bouncing off of nearby tall buildings/geological features such as gorges and cliffs). this canoften be compounded by the posted cache coords being off as a result of the poster's possible 'inaccuracy'. I am sure tht other cacher's in this forum will explain a little more clearly than I can on the operation of garmin units.

 

Second question: 1 Degree = abt 80.5545 Statute ( 70 Nautical) miles. there are 5,280ft to a statute mile. so there are 425327.76 feet to a degree. divide by 60 minutes gives 7,088.796 feet to a minute and 118.1466 feet to a second. 1/10th (.1)minute = abt 709 feet . 1/100th (.01) min = abt 71 feet and 1/1000 (.001) min = about 7.1 feet. World atlas.com and Distance unit conversion source

 

If your unit can handle decimal seconds, then .1 sec = about 11.8 feet , .01sec = about 1.18 feet.

 

Yet another reason why I prefer to use Uniform Transverse Mercator (UTM) coordinates. everything is already based on meters. no nasty mucking about to remember conversions and precision to 1 meter ( 3.25 feet)

 

Have fun.. thanks 4 caching

Edited September 28, 2004 by virgo91967

[+Thot]

using WGS84 and the traditional degree/minute/second notation, how far in feet is each division (i.e. 40d 42.400 is how far from 40d 40.410?)

40d 42.400 is not degree/minute/second notation. It's degrees & decimal minutes or 40 degrees, 42 and 400 thousandths minutes. In degree/minute/seconds it would be 40d 42m 24s or 40° 42' 24"

 

One thousandth of a minute is about 6 feet. The difference in the two latitudes you gave is 10 thousandths of a minute or about 60 feet.

 

Are lat and long the same in feet for a given measurement in seconds?

If I understand your question the answer is yes. There are about 100 miles in a second, so X seconds will always be about 100X miles

Edited September 28, 2004 by Thot

[+Robespierre]

If you're within 50' of a Virtual and you can't identify it, then it's probably one of the virts which got the whole genre in trouble.

[+Prime Suspect]

Second question: 1 Degree = abt 80.5545 Statute ( 70 Nautical) miles. there are 5,280ft to a statute mile. so there are 425327.76 feet to a degree. divide by 60 minutes gives 7,088.796 feet to a minute and 118.1466 feet to a second. 1/10th (.1)minute = abt 709 feet . 1/100th (.01) min = abt 71 feet and 1/1000 (.001) min = about 7.1 feet. World atlas.com and Distance unit conversion source

Despite what worldatlas.com says, the distance is only constant between latitudes, not longitudes. Longitudes converge at the poles, so the distance between them gets smaller, the further away you move from the equator.

[+Kai Team]

As a fellow Garmin owner, I'll share two things that might be helpful:

 

1) the eTrex Legend doesn't have a "built in compass" (the eTrex Vista does). Although your navigation screen may look like a compass, it's not - it's dependent on GPS, and suffers from the inaccuracies mentioned above. Some people carry a separate compass and, when they get to the point where the needle is jumping around, they back off a little and take a bearing and use the separate compass to keep heading in the right direction (rather than doing the drunken bee dance trying to follow the navigation needle on the GPSr).

 

2) The eTrex Legend does use WAAS (Wide Area Augmentation System). There's a lot in these forums and on the Internet about WAAS (see, for example, the Garmin WAAS page), but the bottom line is that WAAS uses a special satellite to decrease the errors, and increase the accuracy, of your GPSr considerably (to less than 3 meters, or about 10 feet, under ideal conditions).

 

However, you have to turn WAAS on (it's not turned on by default). Look for WAAS in your instruction manual for the Legend, and it will tell you how to turn it on. The GPSr also has to acquire a WAAS satellite, which sometimes takes a while. So, turn on WAAS and then leave your GPSr turned on and in an open area (with a clear view of the Southeast, which is where the WAAS satellite for the US is) for a while so that it finds a WAAS satellite. With my Vista, I routinely get accuracy down to 7 feet with WAAS enabled.

 

Between WAAS (which will accurately measure your distance from the coordinates) and usng a separate compass to maintain a heading as you get closer, you'll get a lot closer to were you want to be!

Edited September 28, 2004 by Kai Team

[+saguaroastro]

As stated earlier in this thread, 1 minute of latitude equals 1 nautical mile or 1.15 Statue miles. As for longitude, since the meridians converge at the poles, 1 minute of longitude covers less linear distance the further you are from the equator. To figure it out simply multipy the number of minutes of Longitude by the cosine of your current lattitude and that will give you the linear distance in Nautical miles:

 

i.e My home latitude (Phoenix, AZ) is N 33 40 minutes or N33.6667 to figure out the linear distance 15 minutes of longitude covers multiply 15 * .8392 (cos 33.6667) to get 12.48 Nautical miles the same angular distance in New York City (lat 40 45 or 40.75) would be 15 * .75756 or 11.36 miles.

 

Hope this helps

 

SaguaroAstro

[+fly46]

As stated earlier in this thread, 1 minute of latitude equals 1 nautical mile or 1.15 Statue miles. As for longitude, since the meridians converge at the poles, 1 minute of longitude covers less linear distance the further you are from the equator. To figure it out simply multipy the number of minutes of Longitude by the cosine of your current lattitude and that will give you the linear distance in Nautical miles:

Do you come with a translator?

[+GreyingJay]

I understood him, I just figured I'd never actually do what he suggests. "Simply multiply by the cosine", indeed. Like I spent 4 years taking calculus classes in engineering in university so I could do it all again now.

[+saguaroastro]

I understood him, I just figured I'd never actually do what he suggests.  "Simply multiply by the cosine", indeed.  Like I spent 4 years taking calculus classes in engineering in university so I could do it all again now.

Trust me when I tell you, I'm not about to try to figure Cosines in the field. It seemed that there was confusion over the fact that the distance covered by a minute of latitude is not the same as a minute of longitude. Franlky it's just nice to know info.

 

BTW Cosines are a part of trigonometry, not calculus.