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Emlid in UK for survey: accuracy/post processing questions

#1

Hi all, looking at getting a Reach RS+ base and rover for archaeological survey work in UK and have a few questions.

I’d mostly be surveying excavation trenches and structures. Spot heights, extents, baselines etc. How easy is it to set up codes for types of point?

Most sites I would be there for at least half a day so could set up base station straight away. Most sites will not have accurately georeferenced survey points, and many won’t have good mobile covereage, let alone mobile internet. What would be the options for getting towards centimetre accuray? Base station will generally be no more than 100m from Rover.

I’ve looked at ‘Ways to set the base’, is ‘Post-Processed Kinematic’ the best choice, how do you get the RINEX logs? There appears to be one in my home town. How straightforward is the post-processing, are there step by step guides?

I understand that the Reach takes points in WGS84, how easy is it to convert to OSGB1936? Especially for vertical Z height to convert to m OD? Is this possible in realtime? For post-processing from WGS to OSGB, are there simple step by step guides?

All help appreciated! Thank you.

#2

Hi!

You can use namecoding, such as B for baseline, H for height etc. For now, it is the only option with the standalone software from Reach. Or, you can use Reach as your GPS, but for mapping use some other software, like Mobile Mapper.

PPK is the answer to your question. If you have a local NTRIP caster station nearby, you can get RINEX out of it and process your base station against it. Then, try to process your rover against base station. This is usually how I am dealing in such occasions.

You should check your NTRIP caster. In my country we have RINEX for free. I don’t know, how is it in UK. Post-processing is quite straightforward. There is a guide from Emlid - https://docs.emlid.com/reach/common/tutorials/gps-post-processing/ . Start from here.

For now with Emlid software it is not possible to convert coordinates in realtime. I would recommend to do everything in WGS84 and then convert all the points to OSGB1936 in some GIS software. QGIS is a wonderful free project you should try. There are plenty of tutorials on how to convert points form one system to another. Height conversions can be tricky.

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#4

Many thanks, that is really useful information.

The coding options sound ok for the first few jobs -there’s a lot to get to know and I think a simple set of codes and a good survey notebook would let me get on with learning the other aspects of the instrument. Does each namecode have an incremental number eg B1, B2, B3?

I will look into the NTRIP and RINEX, hopefully someone in UK can help! Is NTRIP/RINEX free in UK?Would I need mobile internet connection to use NTRIP? Would NTRIP be ‘live’ so I could use it for stake-out as well as surveying?

I use QGIS already, so that is not a problem, I could use a dumpy level for z coords until I have the conversion nailed.
Many thanks for your help

#5

Hi @chiz,

You can use any code that suits you. The default point name is “Point1”, “Point2” etc.

Hope someone will provide you with detailed info about NTRIP in the UK.

As for the real-time Internet connection, there’re 2 ways to work with NTRIP: RTK and PPK.
I’d recommend you to read more about it here (RTK, PPK).

If you’re limited in Internet connection, you can use the workflow @jurijs.jeshkins described.
Otherwise, it’s much easier to connect Reach to the Internet and start getting corrections in real-time.

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#6

Many thanks Andrew, I have read the notes on the EMLID page, I’m just hoping a UK user can help with local details!

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#8

If you post process with OS RINEX data your solution will be ETRS89. Use the OS Net coordinate conversion tool to convert to OSGB36 and OSGM15 geoid heights: https://www.ordnancesurvey.co.uk/gps/transformation/

There’s also a batch converter and offline software you can download.

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#9

Many thanks Rory, had found a batch converter at https://gridreferencefinder.com/

So convert from Lat/Long to ETRS89, then using OSNet to OSGB?

Is the OD height conversion fairly straightforward?

#10

No need to convert to ETRS89 as your post processed solution will already be that if you used OS RINEX data to process your observations. The RINEX data is free and usually available within two hours from here: https://www.ordnancesurvey.co.uk/gps/os-net-rinex-data/

We use two Emlid Reach RS+ units to mark our base and ground control points for RTK aerial surveys, our workflow is:

  • Set up a base mark then position the Reach over it and log RINEX observations for ~30 minutes to several hours depending on the distance from the OS Net station;
  • Download the OS RINEX data (either back at the office or on site);
  • Post process the RINEX data from the base and OS reference station in RTKPost (ensure the output is set to Ellipsoid);
  • Back on site enter the solution coordinates into the base Reach unit, then mark and record the ground control check points with the Reach rover;
  • Back at the office we convert the base and GCP coordinates to OSGB36 with OSGM15 heights using the batch converter.

Using this method for our base and control points provides excellent accuracy, our resultant aerial surveys can be accurate to around 5cm.

If the baseline from the OS station to our base is long (more than 50km) we wait a couple of days before processing and downlod precise ephemeris data from the IGS website: https://kb.igs.org/hc/en-us/articles/115003935351 - there’s the RTKGet app in the full version of RTKLib that can get this data for you. Using the precise data can get you a fix solution instead of float, but does entail two visits to the site with a few days in between.

Hope this helps!

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#11

Rory, thats fantastic! Thank you. 50mm from aerial survey is nuts!

I’ll be doing ground survey, but assume workflow is the same. If I’m in the sticks with no access to RINEX (no internet) then I guess I just do all the post-process adjustment after the survey and just won’t have realtime accuracy

#12

If you have a base and rover, let the base average single for at least 10 minutes and then achieve an rtk fix between the base and Rover. That’s it. No connecting to internet services or subscriptions. If you feel like you’re going to revisit the site again then note the collected base position, set a nail or something in the ground and set up on that point with manual entry of the coordinate the next time you come out. No post-processing required. You can turn on logging and process process after the fact if you are not confident with your rtk fix.

#13

What form of accuracy is your 5cm based on? GCP RMSE, camera correction RMSE or ground stakeout? I feel like people throw these accuracy results around and don’t know what they really mean. The P4 RTK is the most recent and prime example because the accuracies that they state in their literature are complete waste of time when it comes to absolute mapping accuracy. That said, I like the fact that you are using PPK and ground control points. I think there are a lot of uneducated people out there that think just because you’re using PPK or RTK that you don’t need ground control points. Not the case if you want what I would consider survey grade accuracy.

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#14

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