UTM vrs lon/lat

[quantumleap]

I purchased 2 audiovox gmrgps 15x38 units, 5mi 2-way with a gps. One use of the gps, of course is geocaching. these units though, only go down to dd.mm.mm and not dd.mm.mmm. So finding a cache is tough. I did notice though that the unit converted .mm nicely to UTM. From there, I tweeked the coords some what. ex.

N42' 34.87 w 71' 21.61 (gcgfr7) ---> UTM

t19 E 309895 N 4717676, very close to the given 306319 4717008. I take the difference is because of the missing last (m) on my system. can I just use the UTM instead of the lon/lat?

thanks to all

[+Renegade Knight]

Most GPS's that use the MM.MM will go to the nerest meter in UTM and people have said that it is a quick way to increase your accuracy with the old units.

[Kerry.]

Those UTM coordinates aren't even close and certainly nothing to do with any missing (m').

 

There's also a little misconception with this UTM, nearest metre and being more accurate stuff. One really has to look at the underlying coordinate system capability as ALL other precision capabilities will be based on that. Basically it's like being able to only measure to the nearest yard (3 feet) and thinking that one can enter things in feet and it's more precise. Similar reason why many receivers (even with M.mmm' precision) won't accept every metre especially in the latitude.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[dsandbro]

It's the old precision versus accuracy thing.

 

You can interpolate the UTM coords to get closer.

 

===========================================================

"The time has come" the Walrus said "to speak of many things; of shoes and ships and sealing wax, of cabbages and Kings".

[Kerry.]

quote:

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Originally posted by Dave54:

It's the old precision versus accuracy thing.

---

 

At nearly 4000 metres difference it ain't..

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[dsandbro]

quote:

---

Originally posted by Kerry:

quote:

---

Originally posted by Dave54:

It's the old precision versus accuracy thing.

---

 

At nearly 4000 metres difference it ain't..

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

---

 

Like most other people, I just figured it was a typo and let it slide. It is non-sequitor to the original question and thus not worth getting all worked over.

 

You will never get an exact conversion from any coordinate system to any other. In most cases the degree of error is well within accepted tolerances and so safely ignored. For geocaching and use of recreation grade GPS acceptable tolerance seems to be around 3-5 meters or so. 0.001 degree is roughly 2 meters so interpolation is acceptable and easy to do field math. A real nerd will whip out a scientific calculator and convert lat-long to UTM out to decimal point land, but most of us do not need nor care to for simple geocaching.

 

===========================================================

"The time has come" the Walrus said "to speak of many things; of shoes and ships and sealing wax, of cabbages and Kings".

[Kerry.]

quote:

---

Originally posted by Dave54:

.... recreation grade GPS acceptable tolerance seems to be around 3-5 meters or so. 0.001 degree is roughly 2 meters so interpolation is acceptable and easy to do field math. A real nerd will whip out a scientific calculator and convert lat-long to UTM out to decimal point land, but most of us do not need nor care to for simple geocaching.

---

 

Yes the calculator syndrome, Fix 9 decimals and we'll be spot on

 

I'll assume then that the "0.001 degree" is a typo then 2 metres? is there another way to read that one, apart from what it's supposed to be?

 

309895

306319

 

4717676

4717008

 

Those 2 sets of numbers really have no typo similarlity at all.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[dave and jaime]

by my calculations a route made from the three points you listed is 3.63km, so check your conversion technique and input data for errors. as for using the utm, you will appear to get additional accuracy but since the practical accuracy of you gps is only about 15m anyway when coupled with the error of the placers gps you probably wont gain much anyway.

 

people must remember that the gps system wasnt designed to find boxes out in the woods, rather for emergency purposes and in those parameters of use the 15-25m accuracy is fine---i guess if your lost in the woods and you cant see you truck/cabin at about 20m you have more serious issues.

 

bottom line---geocaching isnt as simple as entering coordinates and walking to an point, in the mean time when you become familar with your gps you should be able to get yourself closer than the system specs by being able to 'figure out' what your gps is 'thinking'.

 

'Get to the point---speak English!!!!'

[quantumleap]

thanks to all, the only thing I did was to select the type of conversion. I can select, dd.mm.mm dd.dddd or UTM. I selected UTM and it gave me those coords, I punched in the coords on the cache page and Im going to test it out today. I dont expect to find the box in the woods to the pinpoint, but I would like to be closer than a 100 to 150 foot area of the target. now your talking a 1500 foot search area for the cache. I went out yesterday, (prompting my question) and could not find the cache. It is off trail and in undergrowth and brush. The units I purchased are for camping, my children can take one, and off they go, they can then send us thier coords and we can find them if the need ever occures and keep in radio contact as needed. this is just a side job for the units lol.

[yeahright]

Having a display that reads to .0001* (ddd.dddd) means a north/south precision of 36.5 ft and an east/west precision of 27.6 ft (at 41*north latitude). An area of 1000 sq ft.

 

Setting the display to .01’ (ddd mm.mm) means a north/south precision of 61 ft and an east/west precision of 46 ft (at 41*north latitude). An area of 2800 sq ft.

 

I think the reason many people use UTM is that they can relate better to seeing a distance number (even if it is meters) on the display rather than a lat/lon number. Also, for those units that support UTM but do not provide precision finer than that above, UTM can provide better precision.

 

On my Magellan 315 (not upgraded to 3.15 software yet), my distance precision when approaching a waypoint is .01 mile. That is 53 ft. I’ve been told the upgrade will give me better precision. Therefore, I use my Garmin Legend which gives me a reading in feet to the waypoint. To use the Magellan, I have to compare the target UTM coords to my current UTM coords.

 

This is precision, not accuracy. To say that I have 2.3846 children is a very precise number, but hardly accurate.

 

No matter how precise you think you are, you still have to deal with accuracy. Most units give an estimate of “gps accuracy” or EPE “estimated position error”. These are evidently numbers the unit has calculated from the variation of the received position information. This is an estimate – it has it’s own lack of accuracy.

 

We’ve all observed that when you get to “ground zero” , it keeps moving around. The bearing and distance fluctuate wildly. While standing still you can observe with great precision that the cache is 8 feet away, no, it’s 27 feet, no wait a minute, now it’s 53 feet away, hey now it’s 17 feet away.

 

To be practical, when cache hunting, it has become my habit to stand off about 100 feet while my son starts searching under rocks, bark, limbs or whatever. Over several minutes I watch the distance and bearing display and observe the minimum/maximum distance to the target as well as the minimum/maximum bearing changes. Using a real compass, I then can determine the target area. The cache is usually, sort of, in the middle region of the target area. You have to keep in mind that when the cache was placed, the cache owner’s GPS gave them a position with some inaccuracy also.

 

This approach works well when the target area is in a position of poor GPS reception. Step back to an area with a better skyview and then determine the target area.

 

Still tough to do with a non-upgraded Mag 315. Gotta get that upgraded.

 

Keep working with it and you’ll get the hang of it. Then you’ll buy another unit with better features thinking this is the Holy Grail and that you’ll not have any more uncertainties. But even with better precision, the new one will still have to deal with lack of accuracy.

 

Faster, Better, Cheaper

Pick any two.

[2oldfarts (the rockhounders)]

Just a hint for you to ponder. Forget the conversions and read the numbers right under the lat & long on the cache page. It says "UTM". It's already converted. Happy hunting.

 

Smile, make others wonder what you're up to!!!