Reach RS+ Geoid Corrections UK AOD

Hello everyone,

I’m looking for some help / suggestions about the easiest way to convert the elevations for points collected using a Reach RS+ to ground levels above ordnance datum (in the UK). I understand that the elevations from the Reach use a global geoid system and have looked into various web based converters to approximate ground levels to AOD in the UK (London specifically) however I can’t seem to find one that gives the elevations that I expect based on a topographic survey that I have had completed for this site by a surveying company.

Can anyone help me with this, I saw that someone suggested creating a raster layer for the geoid in QGIS but I’m not sure on the specifics of creating such a layer.

Any help / advice is appreciated, thanks

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You might want to verify what horizontal and vertical datum was used by the surveying firm as well as any surrounding passive control marks. Here in the USA, many Surveyors state this on their final product and also show physical ties to passive marks. Once you have determined that, then you could tie into their survey project site.

Do you mean geoid or ellipsoid? Big difference. If you setup your project in Reachview 3 using default vertical datum, then the output is ellipsoidal, not orthometric (geoidal).

If you setup your project with a different vertical datum, then Reachview should output something based on local geoids that need to be downloaded (by the app).

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Thanks for replying @Gabriel_C, i was using the old reachview app, I didn’t even know there were newer versions that allow you to specify coordinate systems, thanks for letting me know.

I have imported my points from the old reachview app into a project with the ODN height vertical datum for the UK and now when I export the CSV I have columns for ellipsoidal height and elevation both of which are drastically different to the levels from the topo survey, do I need to use ellipsoidal height that I have now to correct the elevation?

I will take the unit with me tomorrow and try using the ReachView 3 app with the ODN height to see if I can survey points close to what they should be as presented on a topographic survey.

Glad to help. Bear in mind that if you’re getting a correction from another base, you need to make sure the base broadcasts its position in the correct vertical datum. So that piece of information is critical.

If on the other hand, you’re using the RS+ by itself for long observations with post processing, then you might find that your vertical accuracy will not be very good. There’s no solution to that as the RS+ is a single frequency receiver.

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Hi Tom,

I just wanted to add that you can check the coordinate systems and vertical datums that ReachView 3 supports for the United Kingdom in our guide.

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L1 only receivers can provide the same accuracy as any GNSS multi frequency receivers. In fact, in clear open sky conditions, they are great receivers. We still have two of Trimble’s 4600 LS single frequency receivers. They have high quality L1 carrier phase and C/A code and were great for static, fast-static (short occupation times with local base) and kinematic surveying.

The only downside to L1 only receivers is occupation time (for geodetic quality) and multi-path. We use to establish high accuracy control networks on large construction sites with our Trimble 4600 LS’s. We would set one or two as bases and use one as a rover. Using the fast-static method for these receivers in clear open sky’s (no multi-path), we obtained <1 cm accuracies in both the horizontal and vertical components after PP using TGO (Trimble Geomatics Office). Occupation times were generally 5 minutes/point and observed in optimum low PDOP conditions. You had to plan your surveys beforehand based on predicted orbits and satellite availability. Using two bases, you had a closed loop on every observed point. Loop closure accuracies were generally 1:500 000 or greater.

Keep in mind this was in the late 90’s. GPS only and Trimble ruled the industry.

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