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How Bad Is My Unit Off?


EMTspud
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Ok, guys and gals- I am VERY new at geocaching. Started today and found my first one- but I really had it quite easy using the clues.

 

Still, I seem to have a problem. I have a Magellan 315- had it for years. Today, prior to going on the hunt, I dumped the memory and started from scratch. I initialized the unit and let it coordinate itself.

 

I then compared it to two sets of coordinates available online (www.geocode.com and www.multimap.com). The coordinates online are *nearly* identical. However my Magellan appears to be way off. I took the reading from my backyard and used my physical address to input into the site.

 

Example: The seconds for both websites (North) are 56

The seconds for both websites (West) are 56 and 55.

 

My GPS shows seconds as: 94N and 92W

 

Can anyone explain this to me, or offer help?

 

--spud--:)

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For a start your comparing seconds and decimal minutes, so things certainly should appear a long way off.

 

56 seconds = 0.93 minutes so apart from using different formats your really where your supposed to be

 

Just need to work in the one common format. Datum that could be another issue but not really related to format.

 

Cheers, Kerry.

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Hmm, although the other guys had good suggestions, I'm not sure they explained it very clearly to someone who described themselves as very new.

 

There are several ways coordinates can be expressed.

 

The websites you were using appear to use DDD°MM'SS" where:

 

the DDD is the degrees, MM are the minutes, and SS the seconds, just like they taught in school.

 

Goecaching.com uses DDD°MM.MMM

 

The DDD is the degrees, the MM.MMM is the minutes, to the third decimal, or thousandths place. There are no seconds.

 

To convert to or from decimal minutes (MM.MMM) and seconds (MM'SS") divide the seconds by 60, or multiply the decimal part (the .MMM) by 60.

 

In your case, you say the website reported 56 seconds. Divide by 60. 56/60=.933

 

Since the 315 only displays two decimal places (MM.MM), rather than three (MM.MMM) in the minutes, your GPS showed .94.

 

It appears that you were not off at all. Also, I believe the 315 can be upgraded to display three decimals of precision behind the minutes. You'll need a cable to hook your GPS to your computer to do this, but I highly recommend getting one. You'll be able to transfer waypoints too.

 

I hope I explained that properly. If you have other questions, don't hesitate to ask.

 

Jamie

 

[edit] fixed tickmark reversal

Edited by Jamie Z
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Hey,

 

I really appreciate the replies. Let me get this straight.

 

For example, my unit reads 45.42.94 x 122.55.92 and geocode.com shows my address as 45.42.55 x 122.55.52.

 

How far off (approximately) am I? Close enough for good geocaching, or should I consider another gps?

 

--spud--:)

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I really appreciate the replies. Let me get this straight.

 

For example, my unit reads 45.42.94 x 122.55.92 and geocode.com shows my address as 45.42.55 x 122.55.52.

 

How far off (approximately) am I? Close enough for good geocaching, or should I consider another gps?

 

--spud--:)

I'll use your numbers to better describe the situation.

 

When you say:

 

"For example, my unit reads 45.42.94 x 122.55.92 and geocode.com shows my address as 45.42.55 x 122.55.52."

 

What you really should be saying is that your unit reads "45°42.94 x 122°55.92" and geocode reads "45°42'55" x 122°55'52"."

 

Be careful you are not confusing decimal points with tickmarks for minutes and seconds. And on that note, I just realized that I reversed them in my previous post. I'm going to fix that.

 

Jamie

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EMTspu-

You can change the datum settings on your unit. Geocaching uses (WGS84) most of the time, unless otherwise stated. (NAD 83) is another one to give you another idea. Also your format is xx° xx.xxx - this is the item you really need to check., especially if you want to download from geocaching.

These are a few basic thing you need to know to set. Usually once set - you'll never have to bother them again.

Welcome to Geocaching!

Rogerwilco :)

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I then compared it to two sets of coordinates available online (www.geocode.com and www.multimap.com). The coordinates online are *nearly* identical. However my Magellan appears to be way off. I took the reading from my backyard and used my physical address to input into the site.

 

Not sure how things worked out for you, but it is hard to say where online addressing coordinates take their readings. You say you took your reading in your backyard, but the address coordinates may be computed at the street in front of your house. Just something to consider.

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....and its also Christmas time. Should I do myself a favor and buy a different model? I can always eBay this 315.

 

You guys are VERY cool and patient. It is most appreciated.

 

--spud--:)

Nothing wrong with the 315. It's a good unit to get started with geocaching. Use it for 3 to 4 months until you get a feel for the game and learn what bells and whistles you might like on your next unit.

A previous poster suggested getting a serial cable to connect the 315 to your PC. That should be a priority for the reasons he mentioned plus the latest firmware version (downloadable from Magellen) vastly improved a few things that benefitted cachers.

 

Cheers, Olar

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The lat/longs listed by a website as a translation of a street address are NOT very accurate - they may be a couple of hundred yards/meters off. If you want to check your unit, find a surveyed benchmark (you can view some on Geocaching.com), set your unit to the same datam as used for the benchmark, and take an averaged reading. A good place to look for benchmarks are near city/county/state office buildings etc.

 

A really neat tool is Expert GPS. With this program you can input a waypoint and download a topo map and an aeriel photo. Stand at the corner of a major intersection and take an averaged reading, then enter it into Expert GPS and download a photo. It should show up on the map exactly where you were standing. I did one standing on my front porch - that's where it show in the photo.

The lat/longs you gave as an example in the forum are in the middle of a heavy wooded area and no house is visible.

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