I've heard for a long time that you shouldn't use EDM for very short distances, but I cannot find any information on the web to support this. I know it has to do with the time between instrument and prism being to short for the gun to process that could lead to some error, but I am not sure if that is correct. For layout or location even, shooting a distance of 5' seems ridiculous to me unless it's just a check to make sure your guys didn't cut 1' when they shouldn't have. However there are times when you may need to shoot that close... Even then, I have seen the couple of guns I have used read the same up close as the tape. Anybody have any info on this? Are modern guns more capable of shooting up close?
EDM vs tape is identical in every case I have checked over the years. Somewhere between three and four feet is probably the shortest shot ever. The prism must be absolutely motionless.
If I recall correctly there used to be a manual distance entry on Wild guns, have not noticed it in years. It was nice for topo features right next to the set up. In general once it is too close for a prism to be practical I use Reflectorless, or a tape in rare cases.
Funny to see some chainman react to the tape for measuring from a TS, like it is somehow less precise or against the rules.
Isn't that funny? If you have the super dooper high dollar machine available, how could it possibly be just as good to use a $100 tape???!!??!!
Trying to explain that to the uneducated is like spitting into the wind. We aren't building wristwatches for fleas.
The total error in an EDM distance will be the sum of the EDM error itself, plus the centering error of the instrument, plus the centering error of the target rod. The EDM error for the Spectra Precision Focus 30 I currently use is 2mm+2ppm (0.007'+2ppm). Through long experience with StarNet I have found that a centering error of 0.003' for the gun and 0.01' for a target rod works well. For a very short distance we can ignore the ppm error and calculate the distance error an EDM shot as being (0.007'+.0003'+.01')= 0.02'. Of course those errors, being random, will usually cancel each other out to some degree but there is potential that any given shot can vary by that much.
So 0.02' error in short EDM shot. Do you think you can tape say 10' to better than +/-0.02'? Most, I think, would expect to where the ground is flat and temperatures are moderate . The errors in a short taping due to temperature and tension, which I have no desire to calculate over my Sunday morning coffee, will be very small. But if you had to employ the plumb bob I think the errors in the 2 methods would work out to a wash, at best. Especially given that most crewman these days have little experience handling the bob.
Almost all EDMs will have a minimum range. They just won't shoot a distance under 6 or 8 feet. This because the return at that range is just too strong. On most older models there was no internal safety that just shut the system down at those ranges. It would just burn out sensors. So an absolute prohibition against short EDM shots was born. If your instrument is newer than about 20 years old that won't be a problem. But the legend lives on.
I've done several "structural" location surveys of the interiors of buildings with an older SET4 130R3 reflectorless TS. The short traverses and multiple checks of corners and frame members were amazingly accurate (better than I expected), and in a few cases were peppered with a couple of taped distances. All the error encountered was less than .02'. A number of shots were less than 10' with some less than 5'; some approached 100'. Basically all the slop we encountered was negligible for what we were doing.
McCracker, post: 332069, member: 9299 wrote: I know it has to do with the time between instrument and prism being to short for the gun to process
I don't think this is an issue. The EDM does not measure time directly as a pulse echo, but rather by comparing the phase of a modulation on the transmitted and received light beams. It is sending and receiving simultaneously.
It averages the phase for a while and then converts that to time units, then with the ppm correction to distance units. It does this with multiple modulations to resolve the ambiguity of the modulation wavelengths (typically 5 or 10 meter finest wavelength).
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I wish I understood the short-range limitation better. I don't think it is always the return signal being too strong. I've experimented with the reflective tape, which returns less light than a good prism, and the prism works closer than the tape. So I'm suspecting it is related to the return beam hitting the sensor or not. Since a prism returns the beam from the opposite side from where it enters, there is a distance between the paths. At larger distances, some dispersion of the beam occurs and smears light all over the sensor field. At small distances, the return may miss the sensor? Any ideas on this?
Interesting stuff. I was thinking it may have been an issue with older guns and not so much with modern total stations. I have noticed that up close sometimes the gun will catch half and half and read some sort of error depending on the particular make. Like Holy Cow said, the prism has to be motionless so it's very possible the return is missing the sensor up close.
McCracker, post: 332069, member: 9299 wrote: I've heard for a long time that you shouldn't use EDM for very short distances, but I cannot find any information on the web to support this. I know it has to do with the time between instrument and prism being to short for the gun to process that could lead to some error, but I am not sure if that is correct. For layout or location even, shooting a distance of 5' seems ridiculous to me unless it's just a check to make sure your guys didn't cut 1' when they shouldn't have. However there are times when you may need to shoot that close... Even then, I have seen the couple of guns I have used read the same up close as the tape. Anybody have any info on this? Are modern guns more capable of shooting up close?
Check some of the older ALTA Standards table of what equipment to use to meet the accuracy requirements. They listed the distance where not to use an EDM, but to measure with a chain (tape).
Check the owners manual.
Mine said no shots under 75'
What gun?
Most spec out to very short distances these days. I have seen strange results when the edm is used under 3 feet (sub one meter).
If you have to shoot that close you are working in a strange environment or need to plan a little better. Sighting that tight is a bear and our S6 will flip out when you try it...
Bill93, post: 332109, member: 87 wrote: I don't think this is an issue. The EDM does not measure time directly as a pulse echo, but rather by comparing the phase of a modulation on the transmitted and received light beams. It is sending and receiving simultaneously.
It averages the phase for a while and then converts that to time units, then with the ppm correction to distance units. It does this with multiple modulations to resolve the ambiguity of the modulation wavelengths (typically 5 or 10 meter finest wavelength).
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I wish I understood the short-range limitation better. I don't think it is always the return signal being too strong. I've experimented with the reflective tape, which returns less light than a good prism, and the prism works closer than the tape. So I'm suspecting it is related to the return beam hitting the sensor or not. Since a prism returns the beam from the opposite side from where it enters, there is a distance between the paths. At larger distances, some dispersion of the beam occurs and smears light all over the sensor field. At small distances, the return may miss the sensor? Any ideas on this?
It has been a LONG time since I thought about the phase-shifted doppler that EDMs use. But I think this description is spot on. At a very short range, the instrument can get easily confused in trying to suss out the point along the duty cycle of the modulating wave (I think I had the right terminology there...hmmm). And, as one other person here mentioned, if the prism isn't held VERY still, the error between successive readings will have too much variance for the instrument to be able to calculate an accurate distance.
I find it funny when I will tell my crew to "just chain it" and they look at me with those blank stares because there's not "just chain it" button on the data collector/controller.
