What to do about heavily wooded sites?

Hi all,

I am wanting to purchase a pair of reach RS2+ for creating site plans to design septic systems. I will often be working in heavily wooded areas, although there is usually a good clearing somewhere on the lot already.

It is my understanding that you can end up with only sub meter level accuracy under heavy canopy with the RS2+. I am wondering if you can leave one RS2+ under the clearing as base, and then use the LoRa with the other RS2+ as rover, and get centimeter level accuracy in the wooded areas. If so, how much of a range can you expect to be able to keep cm level accuracy for?

Thanks for your help!

If not, than I believe the alternative would be 1 RS2+ and one Total Station. You would take your baseline with the RS2+ for cm level accuracy, and then total station you design / stake out off of that baseline, for sub cm accuracy.

I’m thinking that might be the only option for professional quality results…

In wooded areas you cannot get fixed results, or at least it is very difficult and it is achievable in small spots, not necessarily the ones you are interested in.
As you suggested the best approach would be to measure reference points in open areas and use them with total station.
Plan ahead how to distribute and observe the reference points and your points of interests. With respect to other projects spending some extra time planning and creating a well tought layout pays a lot on the long run, both considering total survey time and accuracy.
Massimo

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If the trees are not really tall you can use a really tall pole (6 meters for example)
I have done it once. It’s not the best solution since you have to open and close the pole for every point but it gets the job done.

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Hi @argoskier,

I don’t recommend working with the receivers under the dense canopy. An unobstructed sky view is a key ingredient in getting a precise position with the satellite receivers.

Also, I just wanted to add that the LoRa radio range is up to 8 km when the receivers are located in the line of sight. But if there are obstacles between them, this range may decrease to hundreds of meters. Please keep it in mind if you want to pass correction via radio in the wooded area.

The standalone receiver provides several meters of accuracy only. And even in this case, the unit still needs to have good satellite visibility. So in the forest area, I’d go with a total station. But as Massimo said, you can place some reference points in open areas with receivers. And then use these points with the total station.

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Echoing what others have replied so far –

Both the base & the rover need “pretty good” sky-view conditions in order to get FIX-level (cm) accuracy.

I have done several hours of experimenting w/ an RS2 + cellular modem for NTRIP under moderately-dense mixed deciduous/evergreen tree canopy (average height of canopy = 7 to 10m or 20-30 feet) and report the same spotty mix of FIX + FLOAT results. I’d say about 20% of the time I got a FIX, 70% of the time the points were FLOAT, and the remaining 10% were SINGLE (unusable.) This was in the westernmost forests on the California coast, within ~50 miles of the coast line and at an average elevation of 300-600m (1000-2000 feet.)

Depending on the terrain & the distances & number of points to be covered, establishing control points in open areas using a single RS2 & NTRIP corrections over cellular data or tethering, & then using a conventional totalstation to measure the under-canopy points, is probably the next best thing, albeit far more expensive in both equipment & time spent.

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Even in “light” canopy one has to be mindful that the “Fix” status on the receiver does not necessarily mean accurate results as multi-path errors can be introduced from nearby objects.
Always check (and many times double-check) your results if you need them to be survey-grade

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Thanks everybody!

Sorry to beat this to death, It’s just a little hard for me to conceptualize, since I have 0 experience with any of this type of equipment.

Just to go back to my original question:

Assuming my base is placed in an area with a perfect view of sky… I understand that the range of my rover may only be a couple 100 meters, but could I get cm level accuracy with my rover in whatever that range happens to be (depending on how many objects are interfering with LoRa)?

Thanks again!

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You have to be concerned with the rover also getting a clear view of the sky.
A wooded area is not suitable to get survey-grade results
The tree canopy will create interference between the satellite signals and the GNSS antenna resulting in errors.
Errors are the surveyors enemy.
I have repeated measurements to the same point getting a fixed solution in a Pine forest and the two results differed by almost 1m.
If your error tolerance can accommodate that level of (in)accuracy and greater then by all means use the equipment under tree canopy.
I recommend that you determine what your error tolerance is and familiarize yourself with your equipment and its limitations before using for serious work.

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I see. Ya, in retrospect, that makes total sense of course.

Thanks for the help!

@argoskier,

Just wanted to confirm Adrian’s words. The rover calculates the precise position using observations of the same satellites from the base and rover. Because of that, a clear sky view is vital for both receivers.

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I used 2 RS2 receivers in a tropical rain forest. I can ensure you that you’ll still be able to get fix upp to arround 3 km from the base if you set the base in a clear and wide space. Beyond 3 km you’ll start having “floats” and rarely “singles”. But after doing post processing (ppk) you’ll be surprised with the results , you’ll get a huge percentage of “fix”.
Give it a try for a total station or a drone will even be harder to use in search environments.

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Awesome. That sounds promising, thanks!

Just remember that fix doesn’t necessarily mean precise nor accurate.

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Right. So what does Fix mean then?

In a nutshell it means a theoretical precise & accurate measurement

To rephrase, it’s a position that’s estimated to a high degree of precision, but based on the possibly inaccurate information that was available to calculate with.

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@argoskier,

Fix solution status means that the receiver was able to resolve ambiguities in the position calculation.

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Aha, I see, thank you. So If a Fix status means:

“that the rover using corrections from the base resolved the ambiguities in its positional calculation and achieved the solution with the centimeter-level precision.”

…then how might we still end up with inaccuracies in a wooded area? Is it because we may have interference from the actual physical objects between the base and rover that the ‘Fix’ status is of course unaware of?

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See my opinion here on GNSS multipath (I e. woods, buildings, etc)

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