Hi all. I come from a Remote Sensing / GIS background and do not have experience with surveying. So please forgive me if I am missing something obvious.
I have a specific scenario for which I'm struggling to find the best solution, and was hoping to obtain help in this forum.
The goal is to perform vegetation measurements of an area, using a method called "star transect".
The current procedure we have, is to go to a pre-determined randomly generated location, using a regular consumer GPS. The accuracy of a few meters is not an issue.
Once the location has been found, a picket is hammered down, marking the center of the plot. From this center point, measuring tapes are placed on a N-S, SW-NE and NW-SW directions, extending 25m each side of the center. Photos attached.
Once the tapes are placed, vegetation measurements are taken at one meter intervals along the tape, for as total of 150 measurements (3x50).
Placing the tapes trough the vegetation takes up a considerable time, so I am looking into more efficient ways to find the exact location of the one meter interval points along the transect. Ideally I would like some kind of mobile device that gives a centimeter accurate distance and bearing from the center point.
Is there any equipment suited for that ? Laser measuring devices won't work because of the vegetation and no direct line of sight.
Is RTK GPS a good option ?
I'm not sure, because I need a high accuracy relative location from the center point, while moving the pole.
I've seen that some RTK GPSs can take 40 seconds to record an accurate position. Does this means that the I would have to wait 40 seconds for each measurement, only to find out that the pole should be 5 or 10 cm from it's position ?
In this case it is not more efficient than the tape: it would take at least 150 x 40s = 100 minutes, hypothetically considering I place the pole in the correct spot at the first attempt, at each point)
I do not care about absolute positioning, only relative to my center point.
I've been researching for things along the line of Radar distance measurements and RFID location systems, but haven't found anything suited for my needs do far.
I would be very thankful if anyone could point me in the right direction.
RTK solutions should only take a few seconds for a measurement, as long as the link is strong to the base station - I use a cellular link to a real time network which charges a yearly fee ( so you would need good cell coverage) - otherwise you will need two gps units and maybe a radio link - some come with a 1000 foot radio that might work well - a used base and rover unit might cost about 20K for the survey grade units
another solution might be use a drone and get a high resolution photo that you can plot the vegetation in cad and get all the measurements along with an archive of the site at that time - might work well for a desert environment, but tall trees could cause problems with the canopy covering other plants
If line of sight is possible ( a prism staff can be about 20 feet high ) a total station might work well - placed at the center, it would provide distance and the angles needed for layout too- if two people are there, a robotic unit might not be needed - older units can be less than 10K with a data collector
On the KISS side - there are many laser distance meters these days and with a good target ( maybe a 4x4 post painted white, or some paper plates on a pole) - line of sight is still needed but a unit might be $800 for one that reduces to a horizontal measurement using a tilt sensor - using a tripod mount you can get measurements out about 100 feet - set the tripod at the center and measure radially out to targets
Just some thoughts
Ross Kinnie PLS
Total station would do it, don't need anything fancy. Looks like you have free labor so no need for a robotic unit, do not need a data collector or cad package, just something that can collect the data and do cogo on board.
If line of sight is not a problem, and you have all those people that I see in your photo helping out, I would use a high-quality handheld EDM paired with a good compass. Set a high-visibility pole/stake at the center point, then use the EDM to set all the other endpoints around the compass rose with similarly visible poles.
Assign lines to team members, have them use either (1) a meter stick and flip it end over end, or (2) a distance wheel, and walk from their endpoint stake back to the center point. I would imagine this is at least as good as what you were already doing with respect to alignment.
Leica makes some excellent distos that are around US $200-400.
If you really want to nail down the accuracy and cost is not much of a concern, a robotic total station with tracking would work really well.
Fascinating, assuming that this site describes the total job: https://data.auscover.org.au/xwiki/bin/view/Field+Sites/Star+Transect+Protocol+Web+Page
Out of curiosity, what percentage of the total time for a site is taken up by laying out the tapes? The rest of it seems pretty tedious, but there's no obvious way to save time there.
You may not be getting centimeter accuracy with the tapes. The procedure does call for measuring slope, but there seems to be some error in that process as well. And getting 100 points one meter +/- 1 cm apart is a lot to ask for with this set-up.
You may not need the 1-cm accuracy requirement, sighting the directions seems to be somewhat loose, so something like a Disto may do the job. Solving the line-of-sight problem would be good education and well worth the effort.
It may be just as time-consuming as the tapes, though, so more crew training in laying out tapes may be a better answer.
GPS RTK is made to do what you need, locate the center point, make a line each angle you want and start walking down the line locating at each distance, you don't need to lay out the tapes, the lines are virtual. locate as fast as you can walk, one man crew.?ÿ
Is it just the camera angle??ÿ Those lines don't look 60?ø apart.
Do the tapes laid out from the center point have to be exactly N-S, E-W, NE-SW, etc.?
Pick a center point that has the most clear views through the brush, lay the tapes along the clearances and do your measuring from them, as usual.?ÿ The trick is to locate what direction the tapes are laid out at.?ÿ Easily accomplished with a little trigonometry employing the Law of Cosines. Pick a distance along any two tapes and measure the distance between them. (Say, for example, you measure between the 8-meter mark on one tape to the 10-meter mark on the adjacent one and get 4.3 meters; the angle between the two tapes is 25?ø34'20".)?ÿ Do it for every pair of tapes until you complete the circle for a check.?ÿ It sounds more complicated than it is.?ÿ How you establish which tape points "North" can be approximated by the shadow of the center pike at noon.
Alternately you can locate the directions with your compass.
@bill93 In the photo there are more than just the usual three tapes I mentioned in the post, as we were doing some trials.
I don't have free labor, and the regular field crew is usually two people.
Vegetation can be much denser than the photos, as I said in the post, there is no line of sight.
Do the tapes laid out from the center point have to be exactly N-S, E-W, NE-SW, etc.?
The spec linked earlier calls for 60?ø spacing, so NE-SW etc. isn't quite correct.?ÿ I didn't notice an angle tolerance but would guess a very few degrees.
It seems to me that 2 plumb bobs and a fully supported tape could be used to drop a flagged nail on line every meter in any chosen direction.
Get a machete and whack all those bushes outa the way then voila easy peasy?ÿ
Transit and stadia.?ÿ Use a 25' fiberglass rod. Total investment three figures.?ÿ This is how most of us would have done it before we evolved into the electronic gadgetry wonks that we are.?ÿ?ÿ
Stadia would be fast and accurate, easy with two guys and radios. Only drawback would be getting the distance out, that would take some skill.
