I’m looking for a way to accurately (within say 5 feet or so) measure a distance of a mile or longer easily.
I will have a second person at the far point so it can be a solution that requires
two devices.
I need it to be fast. A measurement should take less than 60 seconds to perform.
It should be accurate to within a few <5 feet per mile.
It should be highly portable.
It should allow me to take many measurements per day.
It can cost up to about $2k.
Anyone have any ideas? I thought I would just go get one of those laser ranging devices that hunters use but those and similar range finders all top out at around 800’-1000’ and I need a single measurement to work for up to at least a mile.
So then I thought that there must be a way to send a radio signal to a receiver that sends it back and figure out the distance between the two devices by how long it took but my google searches found nothing.
Laser rangefinder + corner reflector? The effective range is dependent on the distance from point A to point B but also on the reflectivity of what you’re aiming at.
We use a device called a total station for measuring arrow distances shot in flight shooting. Setting up might take a few minutes but once set up measurement can be taken rapidly and are accurate to inches.
There are various models of laser or infrared range finder which might work to the mile , or even 6000 feet… so its a bit dodgy, but the lesser ones are $500 and more expensive $1500 … so perhaps the more expensive one is going to give you the range … check reviews that tell you if the quoted range is “ideal conditions” (no dust/humidity haze) , " ok conditions" (typical summer hazyness) or “poor conditions” range… I guess you just need ok conditions to work.
you get a lot of haze on a humid day in summer.
Over a distance of a mile or more, the main problem is going to be beam divergence. A beam that’s 1 mm across when it leaves the laser will necessarily expand outwards due to diffraction; if you do a back-of-the-envelope calculation, you find that this same beam will be about 3 feet across by the time it gets a mile away.
To the OP: the old-fashioned way of doing this would be with a theodolite and triangulation; but that probably fails on the “quick” and “cheap” fronts. If you’re lucky, maybe you can find an old-school coincidence rangefinder; I don’t know if they’re made any more, but they would be perfect for your purposes.
Use two ordinary smartphones with ordinary GPS receivers. Both sides take a reading at the same time then apply the appropriate surveyor’s formulas.
The nature of GPS is that although the absolute position accuracy isn’t down to 5 feet, the differential accuracy is. Said another way, at the specific locations and time you take your readings both positions will be offset from perfect reality by up to a few dozen feet. But they’ll both be offset by very close to the same amount in very close to the same direction.
You can improve your accuracy by using two phones of the same model. Or by taking a bunch of readings with the phones side by side over the span of a few days and determining what the systematic position bias is between phone A & phone B.
You probably need a dedicated GPS app that will expose the full precision of the position value to the max available decimal places. But it doesn’t need to be an expensive fancy app; the position derivation is done by the phone OS; you just need the app to display it in all its highly precise but not so accurate glory.
Now, now, don’t discount the problems with seeing the target from a mile away and aiming the laser at it. (I’m assuming that the OP knows the area well enough to know if there actually will be clear, unobscured lines-of-sight over a mile or more–flat land, no houses, no trees?)
It’s been answered already, but as for distance, it can, indeed, be affected by mirage effects and thermal gradients. Nevertheless, time-of-flight military laser distance devices can measure to 25 kilometers, which is over 15 miles.
Divergence does indeed happen, but you can minimize the effect by expanding the beam to begin with (the far-field diffraction angle is inversely proportional to the beam size at its narrowest). In any event, you don’t need to capture the entire beam. The main problem is that the beam becomes weaker as it spreads out. You will need a retroreflector to give you a good return at the other end. That will also ensure that you’re not confused by reflections from other surfaces (unless they’re closer, they won’t be as strong). narrow-band filters and use of beam modulation can help distinguish between the reflected laser light and other light sources.
This was my first reaction, too.
A mile is too far to aim at a target without telescopic sights (such as expensive professional surveying instruments.)
It would interesting to know more details --what are you measuring, and why?
But I would suggest not trying to use direct measurements. Use mathematical coordinates, and the Pythagorean formula you learned in geometry class.
Buy a handheld GPS unit for $100, similar to this one.
Change the default display from latitude and longitude to “grid coordinates.”
(Lat and longitude are measured in degrees and angles, and gets complicated. Grid coordinates are measure in feet and inches, or meters if you prefer, and the math is simple.)
Read your North and East coordinates at the start point, and at the end point of the line you want to measure.
Use a cheap calculator to solve the Pythagorean formula like you did in school years ago.
It’s easy.
(if you have a math phobia, and don’t mind me treating you like a child,read on.
If not—stop reading here—so you won’t get angry with me for insulting your intelligence. )
Here are the steps for using the calculator:
Subtract the North coordinates from each other, call it N
Subtract the East coordinates from each other, call it E.
N times N
E times E
add the two and hit the square root key. The result is the distance you measured…
At 1 mile, a 6 foot tall person would look about as high as 1/7th the height of the full moon. Let’s say the target is reasonably portable, say 2 feet. That means it will be around 1/21st the height of the full moon. Not only will he need a telescopic sight, but will need a tripod with fine adjustment controls.
The principle two errors in GPS are selective availability and ionospheric delay. The first of these is currently disabled, and we are told never to return. The latter is something we have to deal with.
The problem with both SA and the ionosphere is that they impose unpredictable and varying delay on the the signal - and thus cause problems with accuracy. However the delay is the same for receivers that are nearby to one another - where nearby is defined in global terms. So a mile odd is essentially next to one another. This means that the principle source of error cancels out when you are trying to work out your distance.
Other sources of error may still be an issue. If there are a limited number of satellites visible in the sky - and in particular - if those that are visible are not well spread out across the sky you will get errors. For instance if the satellites you can see are all in a line across the sky you will get good accuracy in position in the direction of that line, and very poor accuracy at right angles to that line. If your view of the sky is occluded - ie in a city, valley, forest, etc, you will thus have worse problems that you might expect. Another problem that can affect accuracy is multipath reception. This is where the signal bounces of nearby objects and the receiver gets multiple echoes - which can cause cheaper receivers problems. Again more a problem in cities, and rugged or mountainous terrain.
The receiver chips in mobile phones are about the bottom of the heap. Dedicated GPS navigation receivers have a range of smarts that smart phone GPS systems can only dream of. They will acquire lock on the signal very quickly (where smart phones almost always need the assistance of nearby cell towers to hep them along - rendering them near useless outside cell range) are much more sensitive, resistant to multipath, and provide a significant number of other remarkable tricks that make smart phones look like a kid navigating with a magnetised needle on a string.
$2k budget should get you in on the ground floor of a pair of quite capable build for purpose professional GPS receivers. I haven’t looked at the current offerings, but high level chipsets are now supporting GPS, Galileo, Glonass, and maybe even BeiDou. That makes for a lot of satellites, and a lot of accuracy. Access to the Galileo high precision capability is another matter, but may become an option.
Thanks for all these suggestions. This was all great information. I don’t know how I missed these rangefinders rated for that distance though I do notice that those websites are all the kind that say “Call Us!” for prices, lol.
But it sounds like counting on the differential between two GPS receivers to be close enough is the easy-peasy way to go. Thanks! I’m glad I’m stumbled onto these forums. Who knew places like this still could be found!
(FWIW, I work in Film/TV and this for a complicated effects shot that we’re trying to do. So all of the speed and simplicity requirements are there because everything on set has to be incredibly easy, fast, cheap and good - pick four.)
For what it’s worth, the really high-quality GPS measurements depend on the errors being similar for similar locations, too. Basically, in addition to the portable GPS unit, you also have a GPS unit mounted in some permanent installation with an already precisely known location. You measure your location on the portable unit, and at the same time the fixed installation measures its location. It computes how far the GPS is off for it, and transmits that information to your portable unit, which applies the same correction.