Coordinates, Altitude and Scale Factor need explanation

Looking for a surveyor to explain this to me.

So imagine I measure two LLH coordinates on top of a very tall mountain, and then chain the distance between them to get the distance.

Would the actual chainage distance be longer than the exact same LLH coordinates measured deep in a mine below the mountain? It must be, we are measuring arcs right?

Is this number on plot plans accounting for this difference?

How do you reliably measure a square on a steep side hill, with the LLH coordinates diverging as you gain altitude?


Yes, the difference in elevation would make a difference in ground distance, the “Combined Scale Factor” helps minimize the error, but it can’t remove all of it.
If a square is laid out with it’s north and south sides on parallels and the east and west sides are at right angles with the parallels, the sides will not converge. If the sides are set on a true north-south lines then the sides would converge, but then it would not be a true square.

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A contracting Surveyor’s worst nightmare. Absolutely unnecessary unless you are traversing miles and miles. I understand why it’s done but what I still can’t accept is why shooting with an RS2, flying a drone or simply importing a Google Earth background into Civil 3D or Carlson all comes in at the same place and the CAD is a scale factor up… and to drive you even more crazy sometimes it’s not and all data is coordinated.

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Ja, it can be a pain for sure.
In the clip below I tried(my best) to visualize the scale error with UTM by comparing it to NTM (close to zero scale error). Its not LLH but it might give some insight and how bad it can get.

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We used to break chainage on steep hills (meaning keeping the chain as level as possible) so can be quite a difference in distance if you simply follow the ground.

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The youtube video at about 8:00min really explains it. So it sounds like in a smaller flattish area the error could be about 10mm/100m. But depending on elevation it could be more.

So of course this goes back now to driving my tractor in a straight line. If I have a field 2km long the row would then possibly be 200mm out of square without any major hills. If the current NMEA guidance system does not account for this, the line would not be truly straight especially if there was a large hill along the path.

Since any error is handled in the same fashion by the guidance system all of my rows will line up side by side regardless, but have and almost undetectable bend to them.

Again I am amazed that surveyors can find anything precisely twice, measuring eroding mud, on drifting plates, in hilly terrain. :exploding_head:


I think, as with all other professions, you are good at your job if you can hide your mistakes. Like its meant to be like that.
Who`s going to argue over mm/cm with a survyor in the the field? :sweat_smile:


Hi there,

Shouldn’t guidance systems calculate orthodromes for navigation? I mean, taking the Earth roundness into account and slightly changing a heading to keep the line straight.

I’m not sure tractor guidance systems use this method as distances are not that large, and the error is negligible. But I’d do that if I was a developer :sweat_smile:

I am sure the big dollar systems do, not sure about open source at the moment.

But even the open source guidance drives ridiculously straight looking at the results. The longest a field could possibly be here is 2miles. Just trying to make sure AgOpenGPS drives straight as modern science will permit.

But eye vs actually measuring are two different worlds.

I think the only people who know for sure are Australians, the fields out there are comically huge.


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