One Quick Question

[+Neweyess]

Today I flew from NC to Scottsdale, Arizona on business. I had a short time before dark but I took a walk to combine exercise and hoped to find at least one benchmark about 0.6 miles from my hotel. When I got to the area my GPS indicated, the street intersection didn't match the description on the datasheet and it also got dark before I could find the benchmark.

 

Now I realize the location is scaled and not adjusted, but could it be I need to re-initialize my GPS unit for this location. (I have a Magellan Sport Trak Map). I thought that the initialization would only make it find the satellites faster when you changed your location pretty far. Once it found the satellites, (and it did take longer), I thought the accuracy would be the same.

 

But I heard that the clock has a lot to do with the accuracy and I noticed the GPS didn't automatically change for the time zone the way my cell phone does. I don't understand that because the manual says that the atomic clock in the satellites will continually adjust the GPS clock.

 

********

 

I lied about the one quick question. The first one was long and here's another completely unrelated one: Has anyone tried one of those new laser measuring tools. Someone told me you could point the laser to measure a distance, something that sounded like it could be useful for benchmark hunting. He said quantum mechanics applies, as the uncertainty principle is there when shooting photons. The thing shoots a number of them and then takes an average, since quantum mechanics relies on probabilities. You are still left with some inaccuracy but the variation might be small enough that it doesn't matter. Anyone tried one?

 

Neweyess

[2oldfarts (the rockhounders)]

Neweyes, If your GPSr got a lock then the accuracy should be good. Our Meriplat set the clock to our local time and doesn't change (at least I haven't noticed it changing) when we move to a different time zone.

 

Haven't tried one of the laser devices, so can't answer about that. But a new toy is always fun.

 

John

 

edditted for speeling

Edited January 15, 2005 by 2oldfarts (the rockhounders)

[+Neweyess]

Here I found an example of one of the laser tools

[+Neweyess]

Well actually that one says its ultrasonic with a laser pointer. Maybe this is the real thing but more expensive?The real thing?

[2oldfarts (the rockhounders)]

Here I found an example of one of the laser tools

 

We thought those nifty little lasers would cost more than that.

 

They should work for measuring because they use the same technology that the Depth Finders on boats do & those are so accurate that you can "see" the fish between your boat & the bottom of the lake.

 

Oh goodie, John wants another toy.

 

Shirley & John~

[+Renegade Knight]

Looks like they need to bounce off something to get a reading on what's reflected back. That would be great for someone who needs to measure rooms and the like. Or the distance from here to that tree/car/object. But to point it at something presumably on the ground would strike it at an angle (ignoring the rougness of the ground). That would make it so I'd want to test it out first to see if it's going to work like I'd like it to work. Maybe there is enough roughness in the ground to make it work.

[+Black Dog Trackers]

I have one of these things - I got it for Christmas. The laser is not for measuring - it is only for letting you know where you're aiming. The measuring is done with 'ultrasonics' (I can easily hear it clicking). I tried it on a tree numerous times just now and got about 15 measurements around 23 feet, varying by around an inch or two, but with a couple of measurements of 30 feet.

 

Some of the brands state their accuracy - 99%, 99.5%, and some higher. (1% of 50 feet is 6 inches.)

 

It won't work on a slanted surface, like pointing it at the ground at different distances, because the sound bounces away, not back. Many to-reach descriptions mention powerpoles, and these type of measurers would be pretty good for that, unless you were going to have to dig for the mark, in which case I'd rather be using a tape measure. I'd say these are several percent better than measuring by pacing.

[+Black Dog Trackers]

A review of products indicates 2 technologies - sonic and laser. (The laser pointer that comes with the sonic ones is a bit of a red herring. )

 

The laser models are about 10x the price of the sonic ones and have about 100x the accuracy and 10x the longest distance specs.

[+PFF]

I have an ultrasonic measuring device for use in my real estate practice. While it is accurate under some conditions, I have learned the following:

 

*Target surface must be flat;

*Will be thrown off by windows, curtains, and some fabric wall coverings;

*Sometimes will read an object along the path, rather than going to the far wall, with examples being ceiling fans, doors, etc.;

 

And the oddest situation is what I call the room's "mystery force" which distracts the unit from accuracy. I first discovered this when a reading obviously was incorrect, so I moved across the room, reversed the path, and the reading corrected.

 

(Yes, I was near an air conditioning vent, and the air handler was running. But that's not a valid explanation for someone who loves a good conspiracy theory.)

 

I must admit I have not used the unit outdoors. But based on performance in the more stable environment of a closed room, I don't think I would trust an ultrasonic measurement. I can pace quite accurately with my three-foot stride, and I always have the 100-foot tape in the trunk of the car.

 

-Paul-

 

I was showing some newcomers to the hobby how to prepare

a survey disk for photography. Reaching into my pocket, I pulled

out a Tupperwear salt shaker and announced that I was going to

sprinkle the disk with Monument Dust. As they watched this

solomn procedure, I said, "You may know this substance by its

scientific name---Corn Starch."

[ArtMan]

Getting back to the original question, on my GPSr (the cheal yellow Garmin etrex) the time zone has to be set manually.

 

I believe the time transmitted by the GPS system is UTC or some other zone-neutral time since the satellite sends signals to receivers in many different time zones simultaneously.

 

The receiver certainly could have a table of locations that would tell it what time zone it is in, but you might check your Magellan's manual or look down in the menus to see if there might be an adjustment for time zone.

 

Good luck!

 

-ArtMan-

[holograph]

I thought that the initialization would only make it find the satellites faster when you changed your location pretty far. Once it found the satellites, (and it did take longer), I thought the accuracy would be the same.

 

But I heard that the clock has a lot to do with the accuracy and I noticed the GPS didn't automatically change for the time zone the way my cell phone does. I don't understand that because the manual says that the atomic clock in the satellites will continually adjust the GPS clock.

 

There is a separate forum to discuss GPS technology, but briefly, your receiver locks on 4 satellites to solve for 4 variables: the three spatial coordinates and the one time coordinate. Once it has the lock, it knows the place and time.

 

If you are far away from your last known coordinates (or haven't turned the unit on for a while) the receiver takes longer to figure out where it is and which satellites are in the sky. If you give it a hint beforehand by presetting your coordinates, you make its job easier and it should take less time to get a lock.

 

Has anyone tried one of those new laser measuring tools. Someone told me you could point the laser to measure a distance, something that sounded like it could be useful for benchmark hunting. He said quantum mechanics applies, as the uncertainty principle is there when shooting photons. The thing shoots a number of them and then takes an average, since quantum mechanics relies on probabilities. You are still left with some inaccuracy but the variation might be small enough that it doesn't matter.

 

Well, there is uncertainty, but it has nothing to do with quantum mechanics. Any QM uncertainty is on the order of the wavelength of light, which is so small to be ridiculous. The uncertainty in laser ranging is due to the temperature and humidity of the atmosphere, the inaccuracy of your pointing, and the quality of the device. For about $15,000 you can get a engineering quality device that is accurate to millimeters over a range of a couple hundred meters.

 

For about $500 you can get a true handheld laser measure that may be accurate to 1/8 inch up to about 100 feet.

 

For $200 or less, you are getting an ultrasonic measure that uses a laser for pointing.

[+Neweyess]

Getting back to the original question, on my GPSr (the cheal yellow Garmin etrex) the time zone has to be set manually.

 

I believe the time transmitted by the GPS system is UTC or some other zone-neutral time since the satellite sends signals to receivers in many different time zones simultaneously.

 

The receiver certainly could have a table of locations that would tell it what time zone it is in, but you might check your Magellan's manual or look down in the menus to see if there might  be an adjustment for time zone.

 

Good luck!

 

-ArtMan-

Yes, I can change the time zone by re-initializing and entering the new one. What I really wondered is if that makes any difference in the accuracy. I heard that the clock accuracy makes a difference but I don't know if that's time zone dependent or the neutral time that you mentioned.

 

I also knew that initializing would speed up the time for finding the satelites in a new area but it really didn't take but a few moments anyway. I just wondered if it's worth doing when you travel long distances from home for the sake of GPS location accuracy. Otherwise the clock doesn't bother me and I wouldn't want to change back and forth for nothing.

 

Neweyess

[Bill93]

If the unit locks on the satellites, then it's going to give the best it can with current reception conditions within a few minutes, regardless of what it started out with. Re-initializing may have an effect as noted on initial acquisition in a new area, but I can't see any reason it would help otherwise. Once it locks on the satellites it is doing far better than any hints you can give it.

 

If you change the time zone setting, it will display time for a different zone, but that doesn't affect its knowledge of precise time internally. It effectively knows UTC at Greenwich regardless of which zone you display.

 

If time is 'way off when you turn it on it will take longer to find the satellites. But when the unit is tracking several satellites, it knows time to within a few nanoseconds. The speed of light is roughly 1 nsec per foot, so you can see how time accuracy is essential for position accuracy. Much better time accuracy than you can manually set. There is another multiplying factor depending on the angles to the satellites, but we don't need to go into that.

[+Neweyess]

If you change the time zone setting, it will display time for a different zone, but that doesn't affect its knowledge of precise time internally.  It effectively knows UTC at Greenwich regardless of which zone you display.

OK. That it does it for me. Thanks to everyone who relplied.

 

I went back out and found the benchmark that raised the question for me in the first place. DU1099 The scaled coordinates were a whopping 1.7 miles off.

 

Neweyess

[evenfall]

Paul, And others wishing to use Electronic Measuring devices.

 

In your case I am assuming you are using a product from Calculated Industries Paul?

 

The Physical problems associates with Sound and Air Pressure can affect accuracy. So can extraneous noise and indirect reflections or sonically diffusive or absorptive materials. To aid with the combat of "mystery force" on items like this it might be useful to carry a small piece of Glass with a reflective adhesive tape along to use as a reflector. The Principal is similar to the ones used with EDM in Surveying. We use a prism mounted to a prism or range pole as a target when we measure angles and distances and the prism or reflector assures we get the measured info back to the instrument accurately. We go together with both measurements at once to accomplish a trigonometric leveling and positioning. If you have a light based tool your self, the same principle should work, if you are using a tool which is truly Sonic, the hard glass should help accuracy by reflecting the sound directly back to where it came, and a quiet environment when attempting to use a sonic tool may help too.

 

In the end the physical properties of light are much more stable than sound if that helps anyone in a future purchase.

 

Good Luck!

 

Rob

[+PFF]

if you are using a tool which is truly Sonic, the hard glass should help accuracy by reflecting the sound directly back to where it came

 

Great suggestion, Rob. I might give that a try. I recall those hands-on science museums where you stand in front of a curved wall and whisper to someone 30 feet away across a crowded room (who is standing in front of another curved wall). Awesome display of how to focus sound!

 

Because I often am working alone, I use the 100-ft tape, sonic device, and a distance wheel. (The wheel is most accurate on paved surfaces, and least accurate over grass/dirt.) But after reading about the LASER devices (and I mean the real ones), I know what I want for my next new toy. I have a LASER level, and it's really amazing how concentrated the light is at 100 feet!

 

-Paul-

[evenfall]

Hi Paul,

 

For the most accurate measurement at the, well, moderate cost, a Nylon Clad Steel Tape which measures feet in tenths would probably last the longest remain the most accurate and take the wear and tear of being tensioned taughtly while measuring under repeated use.

 

The other instruments you have, too have their place. I use the wheel type units quite often when figuring earthwork volumes for bank yard quantities, cut to fill ratios and basic area calculations where I really only need a ball park idea. The larger the wheel the higher the 'relative' accuracy. These work great if you need to make rather lengthy linear measurements, where a close enough measurement will get you where you want to go. Most people probably realize this, but a hint for keeping tapes in better condition is to wipe them off as you roll them up or after use. Sand, Grit and Moisture can soil, abrade and otherwise damage any tape measure.

 

Rob

Edited January 16, 2005 by evenfall