Accuracy: the the point

Reach Module / M+ / M2 Getting started
1

Allright, so the Reach has been existing for some time, and some people have tested it.

If you use a single Reach and process it with NTRIP data from my country’s VRS data, and you would use it like a surveyor to measure points quickly. What’s the accuracy you’re REALLY getting?

And how much bigger does it get if you setup a second one as a base station, on an unknowd point ?

2

Here is a good topic about Reach precision:

If you setup a base station at an unknown point you will get accurate relative position, but to get accurate absolute position you will need to define the base coordinates.

3

I’m not sure about NTRIP or VRS and I am not an expert, but I do use Reach to log data at stationary points and then post-process the data. In doing that, I get good accuracy.

For example:

  • I made a triangle between two known points (numbers 1 and 3) and an unknown point (number 2).
  • The two known points have published latitude/longitude/height accurate to 1mm and they are 9km apart.
  • I place the base station at the unknown point (2).
  • I take the rover to known point (1) for 20 minutes.
  • Then, I move the rover to known point (3) for 20 minutes.
  • Later I download the log files and post-process the data with RTKLIB.
  • Calculating from known point 1, I find the position of my base station on the unknown point (2).
  • Calculating from the now known position (2) of my base station to the known point (3), I get the result and compare to the published coordinates.
  • I did this exercise twice on different days.
  • On day 1, my result was within 3mm of the published coordinates, and on day 2, my result was within 3cm of the published coordinates.

So, from that exercise, I believe that I can be accurate to a few cm in a triangle with sides of length 9km, 6km, and 5km and I am very happy with that.

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4

How does this work, placing the base station at an unknown point and getting this accuracy?

5

Continuing the discussion from Accuracy: the the point:

Sorry, I left that part out for simplicity’s sake, but here it goes:

When I placed the base station at the unknown point (2), I started in rover mode to get the rough coordinates. I switched to base mode and entered those coordinates in the config page. Remember that I am post-processing with the raw log, so I don’t believe it matters if you enter the coordinates, but that is how I did it.

Now this may seem backwards, but when post-processing, I used the rough base coordinates at unknown point (2) in RTKPOST to get a fix for the rover at known point (1). Of course, my fix at the known point (1) was offset from the published coordinates by a certain amount. I took the difference and then I offset my rough base coordinates by the same amount. Now I consider my new base coordinates to be accurate, but I need to prove that they are accurate. So, I used the new base coordinates in RTKPOST to calculate calculate a fix for known point (3). Since that fix came very close to the published coordinates, then I believe that I have proven my base coordinates to be accurate. As I mentioned before, I did this exercise twice.

I’m sure there are better ways of doing it, but that is a discussion for another thread. Can anybody else speak about their accuracy?

6

So could you do the same thing with no known coordinates? I was thinking the other day to get accurate Base coordinates you could have the Base and Rover flip flop from Base to Rover or Rover to Base until it figured out where it was… Would something like this work, such as for when post processing is not an option, the data is needed at the moment it is acquired?

7

No, nothing like that will work. You need to start from a known point somewhere. Whether it is another person’s survey marker, or the known coordinates of their base station.

If you have no known coordinates near by, then you must do the best with what you have.

Are there no accurate survey markers in your area with published coordinates? If not, then the worst case scenario is using 1 GPS to log posititions and average them over time. But you should be able to use a Precise Point Positioning (PPP) service, or get some correction data from government, or university, or a private company.

Once you have an accurate point marked for yourself then you are set!

Keep in mind that you can put your base station in a place with very rough coordinates and do your RTK work with it. Later you can post-process your base station log with third-party data to make it’s location more accurate, then offset your survey points by that same amount for better accuracy.

8

Well, I was planning on using it for agricultural use, out in the country… Set up the base station out on the job site, the 10km limit is not close enough for out here and I looked at public base station maps and there is none that would work for the majority of the time. Is there any other way to get an accurate base point which is accurate enough that it could steer a machine in a straight line, where it needs to go?

9

Morning Guys.

This needs to be stressed. Reach can be very accurate both RELATIVELY and AUTONOMOUSLY.

For it to be RELATIVELY accurate, it needs to adjusted or based on a known point.

For it to be AUTONOMOUSLY accurate, it does not need to be adjusted or based on a known value.

For both operations, you will get cm results, always relative to your base station. It is just that in the case of RELATIVELY accurate surveys, you will be able to coordinate the same place in space with the same value each time you survey. AUTONOMOUS (other survey institutions will use words like deferentially un-corrected or localized) surveys mean that each time you try and measure a point, you will get a different value for it, varying a few meters.

It should be clear, this is not a downfall of Reach. Even the top dollar systems do this system. Part of survey training is to understand how this works and how to decide if the impact of you survey requires it to be based on a known point or if it can be localized.

Weston, from my understanding of agricultrual systems (which is very limited), they do not need to be based on a true WGS84 or known value. As I understand it, they are localised, so if there is a system in place in the field, you will need to compensate for it by measuring on to it. But I think this is unlikly. If the equipment is just required to drive in a straight line, then using the Reach in an AUTONOMOUS survey mode will be sufficient.

Remember, there is nothing stopping you from making your own localised survey repeatable by placing the base on the same position each time you survey and using the same coordinates each time. There is a reason why the base remembers your last coordinates. Using this methodology will give you repeatable and consistently accurate results.

Weston, if you have to base it on a known point, you can really push the reach to get a decimeter value at 50km from a base station. While it will never give you a fix(if it does, it will be junk data), but you will be pretty much close to the value. The other option is to make a series of 8km baseline surveys and move the base each time to the previously surveyed value. I would not recommend this approach as you can introduce big errors and there is no way of checking your final answer.

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10

Thank you very much… That clears up everything for me, I am not kin to GNSS nor RTK and this helped me to understand what I need to know. For AUTONOMOUS modes, how would this be set up, map out the data at one time and then come back later, placing the base station in the exact same place with the exact same coordinates, and offset the deviation from a previously determined point plotted from before?

Thanks,
Weston

11

OK here goes a basic workflow to create a repeatable coordinate system that is localized.

  1. Find a suitable safe position within 5-7km of the extents of the site you wish to survey
  2. Place the base antenna in a safe place that has unobstructed views of the sky, away from EMF etc. This spot needs to be the same every time you do the survey, so mark it out. If its placed on a tripod, make sure that you either keep the height of the tripod/pole at the same height above the ground for each session
  3. Start up the Reach designed to be the base in Rover “single” mode and start it. This will give you a starting coordinate that is not differentially corrected.
    - 3a. I normally send the single solution via NMEA via TCPSVR to my laptop, then to Global Mapper. From there I can average the coordinates to get a better or more true WGS84 value.
    - 3b. If possible, connect to your closest NTRIP server, even if it is beyond the limits of L1 processing, to get a more or less result.
  4. Write down this coordinate. I sometimes write the coordinate down and store it in a waterproof, hidden place close to the base for future reference.
  5. Swap to base mode on the Reach. Enter the coordinate you have just determined.
  6. Start you survey as normal.
  7. It is good practice to place 2 other semi permanent points nearby and coordinate them with your determined system. These can be used as a check to make sure that there are no typos or perhaps the base position has moved or been disturbed. Measure and check on to these before and after you session.
  8. Repeat the process by setting up on the same point as before and entering the same coordinates each time and doing your checks. Doing it this will will ensure you survey is repeatable and consistent.

Good Luck!

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