RTK Base Coordinates

Good Day,

I’ve read that ReachView displays ellipsoidal heights, but what would be the effect if I use the orthometric height for the base coordinates during an RTK survey?

Thanks in advance.

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Very good question.

If the orthometric height does not differ with the ellipsoid height by much then your ability to get a fix should not be adversely affected.

The further away your entered base coordinates are from the actual position, the harder it is to obtain a fix. And I’m not sure if vertical differences matter nearly as much as horizontal differences do.

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Seems for your vertical to be correct, the KNOWN point you use, should be based on WGS84 ellipsoid height when using ReachView, ReachView Survey and ReachView Stakeout.

I.e. the Benchmark record you get that provides the coordinates and the ellipsoid height is what you should enter as your base MANUALLY and also enter your antenna height.

Then either use a third party survey program such as FieldGenius (FG) to load a GEOID file in order to get real-time on-the-fly orthometric height. Or don’t load the GEOID and get ellipsoid height while working, like ReachView. What you see in ReachView will match in FG if using WGS84 for your vertical.

Can also use other software or websites to convert your ellipsoid height from ReachView to whatever you datum, orthometric, etc you need. (i.e. http://www.metzgerwillard.us/geoidge/geoidge.html)

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Thanks for the prompt response mates!

ReachRS+ still gets a fix, even with the entered orthometric height.

Let’s say the published ellipsoidal height is 100, and the inputted height (the EGM08 height) is 200.

Would the computed RTK heights be in the 100 or the 200 range? Would a linear shift (200 - 100) be enough to correct the output values?

From my experience it doesn’t matter what you enter as long as the latitude and longitude are correct (or close), but be wary because the deltas in elevation you are going to get from point to point are not going to match a GEOID survey or an actual land calculated survey.

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?

Why would it not matter what you enter for the elevation? :thinking:

Just in my experience the long baseline is because of the lat/lon. I have done the same thing trying to get relative elevations and had no problem acquiring a fix. As I said though it did not work for me because a side-by-side comparison with my localized Topcon did not report the same elevation change from point to point. Sometimes I was a foot high and other points were 6ft low because of the differences in the Geoid and the Ellipsoid. You would have to be pretty far off vertically to create a huge baseline, but it would still only be a slope distance.

Seems the elevation definitely matters when entering manually the known coordinates AND ellipsoid height elevation AND antenna height. Otherwise the option to, wouldn’t be there?

It’s all part of the 3D space of BOTH horizontal and vertical, not just 2D horizontal in order to calculate ROVERS position in 3D space.

What am I missing here?? Or am I not?

In this case, elevation does matter. The orthometric heights can be calculated from the ellipsoidal heights and the geoid undulation.

Back to the matter at hand, in case the orthometric height was inputted in the base coordinates, would the ReachRS+ output the correct height? if not, will a linear correction be enough (output height + difference between ortho and ellipsoidal heights)?

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Unless you don’t enter the correct known ellipsoid height (WGS84), (along with antenna height) seems your vertical will be off the difference of ellipsoid height and orthometric height… so coming from RS+ (or Reach RTK, M+, RS & RS2) it will not be based on ellipsoid height if you enter a different vertical measurement. I.e. orthometric.

Emlid ReachView uses WGS84 ellipsoid height. (Nothing else currently) So best to stick with that and anything further NMEA streamed to a third party program / app… or use online converters etc as mentioned.

You’ll still get a fix, your elevation will be shifted if incorrect relative to the ellipsoid. (kind of like as if you entered the wrong coordinates for your horizontal, instead of the spot on ones) Everything is calculated solution between the sats and the Antenna Reference Point (ARP) of the BASE and the ROVER.

https://docs.emlid.com/reachrs/common/tutorials/placing-the-base/

Well, it will be an offset to the ellipsoid height and the delta from point to point will be the same as if you had used the ellipsoid height. Either way your dealing with a not true to ground value.

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