# What to do about heavily wooded sites?

When the receiver is in fix, the solution is usually centimeter-level-accurate. However, in forest areas, as Gabriel and Adrian mentioned above, the signal may reflect. For example, from foliage. As a result, the receiver will get incorrect data for calculations. Thus, you need to be careful with measurements in such areas and monitor more parameters, such as:

• PDOP to be ≤ 2

PDOP shows the quality of the configuration of the satellites in the sky. In a forest, PDOP is most likely too high. It has a straight influence on the accuracy you get.

• Age of differential to be about 2 seconds

Age of differential shows the difference between epochs of the base’s and rover’s data. Increasing this parameter means that there are interruptions with the corrections stream.

• RMS

RMS helps evaluate the solution’s changing. So you can notice its instability.

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Aha, thanks Julia and Bryan, that all makes sense.

Clear views of the sky are of paramount importance when using GNSS equipment, especially when that equipment does not cost \$30,000.

Bryan, what do you mean by “closed observation loops”?

Thanks for that information Bryan!

I was just wondering what is meant by 'closed observation loops"?

Where closed polygons represent a “closed observation loops”.

If a square has (4) equal sides, (4) 90° square corners and are measured; 4.00’ x 4.00’ x 3.97, x 3.88’ . There will be a mis-closure in the true linear shape of the square. The angles of measured sides will not = 90°.

This tells you the linear error of closure, where the measurements are not perfect. The measured lengths as well as the angles of the square can be theoretically adjusted by various mathematical means.

Most if not all surveyors perform some kind of adjustment if they have *closed observation loops" of their measurements.

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I see, that does make sense to me in theory. In practice would that mean always taking your last shot in the same place as your first, in order to close the loop?

Yep, think of a closed polygon. How do you know if the polygon closes mathematically without the measurement to close the polygon.

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A closed loop gives a feedback and peace of mind.
You can also have an open loop “closed” on control points, so your survey is not a ring, but a chain measuring some points of known coordinates. This is when your GPS might come to the rescue.
Create some control points in open areas with your GPS, at least at the extremes of your open loop, and use the points to rototranslate and verify yout total station survey.
Massimo

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Can anyone, either pro surveyor or EMLID staff, comment on whether doing PPK on non-ideal GPS surveys with a nominally-high percentage of “FIX” points can or will improve accuracy & reduce the impact of possible multi-path errors?

If possible, maybe even with ballpark estimates of expected range of improvement. No doubt it will be “improved,” but is that just 5-10% improvement? Or could it be a much greater improvement like 50-80% or even higher?

( i.e. GPS surveys done under tree canopy, or anywhere else where multipath is a significant problem)

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Hi @manscuto842,

RTK and PPK algorithms are different. RTK solution depends on the stability of correction flow, while PPK is free of it. Plus, PPK allows playing with the settings. So using it can improve the results in some cases. But the exact changes always depend on the data.

Still, the number of common satellites in view, their location (PDOP), obstacles presence, and multipath affect both RTK and PPK. So when the raw data quality is poor, it will be hard to achieve precise position in any way.

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Many details were already mentioned but some practical tips might still help.

We have done quite a few GNSS measurements in heavily wooded sites and that is definitely going to be a challenge. Both the occlusion and multi-path effects will reduce your available signals and throw off your measurements, sometimes by multiple meters.

Having said that it is still possible to perform accurate measurements with an RS2 under dense canopy.

One strategy for RTK: have your base relatively close which helps with convergence and start in a spot that has at least some small opening in the canopy and from then slowly walk towards the point you want to observe. With a bit of luck the RS2 can keep the RTK fix and you can perform your first observation. Now due to multi-path effects there is a high (wild guess: >50%) chance that this is a false fix. What high-end receivers now do is some patented tricks to figure out if the fix is false or not. With the RS2 you can do this manually though: re-visit the same point half an hour later. The satellite constellation will have changed sufficiently that if you get a fix again and both the first and second fix coincide there is a high likelihood that you obtain the correct position.

The other alternative is PPK. Here the idea is roughly similar, to compensate for the multi-path effects you need to increase your observation time dramatically. If you might only need 15 minutes under clear sky with services such as CSRS to reach sub-meter accuracy when under a tree this might take more than an hour of observations.

Both alternatives are clearly far from ideal and very time consuming but if you only want to measure a few points might be an alternative. Some high-end receivers have better multi-path mitigation technologies, they can’t beat physics but they throw more hardware (channels, processing power) and software (mitigation algorithms) at the problem. Some also come with combined photogrammetry & GNSS features which allow you to measure a point from a few meters away.

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PP is something everyone should learn, whether it’s Emlid Studio or a commercial software such as “Javed Justin 3”.

Short baselines (on project site) along with adequate observation time on station are your friends. Long baselines = longer observation times. PP with closed GNSS traverse loops or terrestrial traverse loops is the only method to truly know and verify your “fix” in high multi-path areas.

When I’m able to go out in the field, I usually have both my M2’s as base receivers with a baseline between them usually <1km. I’m using my RTN rover locating points (property corners, creeks, roads, etc.); I have a closed GNSS polygon for each point whether I have a “fix” or not. Baseline lengths from the rover to the base receivers depend on each project site; usually anywhere from 50 meters to 2 km.

We have Javad equipment here, I’m usually using my Javad rover and base M2’s with the state RTN. Even though I may have a “fix”, I’m always saving my raw data at the rover whether in the woods or not. We use our Javads in high multi-path areas, they are the only receivers that can verify a position in high multi-path areas. These receivers are very expensive, >\$20k. The Javad receivers have paid for themselves on various small and large projects due to their ability for the user to have high confidence in point location while in multi-path areas.

I’ve used my RS2 and M2’s in high-multi path areas and have gotten “fixes” on points; I’ve always revisited these points when using Emlids receivers. I’ve gotten bad and good results in the woods, but I always have the raw data to PP with my M2’s on site. It’s always amazing when verifying the “fixed” field shots PP. Some “fixed” Emlid points have agreed fairly well when PP; <2-3 cm horizontal and vertical. Others not so well, but I have the raw data to PP and to determine the true location. It’s always fun verifying false “fixes”, closed project site GNSS loops usually have closures in excess of 1: 50,00; depending on the length of the baselines.

I’ve said this many times on this forum:

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Thank you for that Alexander,

That certainly is quite refreshing to see some practical (maybe this will work) steps laid out. Love it. Great common sense ideas it would seem to me. Will definitely plan on trying those tips!

Thank You!

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Thanks for these e very useful techniques.
Just to add, I performed a task using similar field techniques a few weeks ago - with the exception of repeating the measurements with the RS2. I was measuring 4 points along a relatively straight fence just on the fringe of a forested area. I managed to get a “Fix” reading on the 3 of the 4 points.
I revisited the the site with a Total Station - Lo & behold one of the points that were previously recorded with a Fix measurement was out by 5 meters. This is a new record error observation for me.
Perform routine checks on your measurements esp when your gut tells you something might be off.

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