Project Preferences - Reference Level

[Anonymous]

I'm having trouble understanding how to setup a terrain using a Reference Level (AHD).

The attached survey shows the FFL of the building will be at RL 38.29, and the lowest spot level on the SS is at RL 36.58

In the Project Preferences/Reference Levels, what do I need to set my AHD Elevation to, -36000? being roughly 500 below my lowest RL??

[Barry Kelly]

Your FFL is best modelled at storey 0 (zero) at a height of 0.
So I would set the AHD level as -38.29.
That way you can reference heights in your model to the AHD and they will be accurate.
Barry

[Anonymous]

Barry I just want to making sure I understand this 100%. In other words, when I'm given a site survey, I would first look for a finished floor level, which becomes my AHD Reference Level " being my -elevation"

Am I on the right track?

[Barry Kelly]

That's what I would do.
I always set my main floor level at 0 and work from there.

If the floor level is above the datum you are using then set the reference level for that datum to the negative value of that datum level (positive if you FFL happens to be below the datum).

Now you can place elevation heights and reference them to the floor level or the datum depending on your needs.

Barry.

[Anonymous]

Ok so I've set the Reference Level to -38290, and clicked on the Mesh Tool and set the depth to 1000.

Is this so far right?

[Anonymous]

Mesh

[Barry Kelly]

Going on your figures you will need to set your mesh height to around 2500 so the base is lower than your lowest spot level.
Add the contours to the mesh and when you set the height for each contour (or spot level) make sure you set the height relative to your AHD.
Then you just need to type in the actual level - i.e. 38000 for the 38.0 contour.

See attached image but ignore the heights I used (just testing it myself).
Barry.

[Anonymous]

Yep comfortable with contours and spots, the part I get confused about is this mesh settings. So that 1000 depth I don't need?

[Barry Kelly]

Make it 2500 in this case.
If you are placing your flat mesh with the top a 0 (which is your AHD level of 38290) then you need to make it deep enough for your lowest level which you say is 36580.
So you actually need at least 1710 (38290-36580=1710) for the height - but make it 2500 so you are well and truly below the lowest level.
Then add your contours and adjust the heights relative to the AHD.
Barry.

[David Shorter]

Personally I wouldn't use this method, although if it works for you then fine.
The downside is that levels cannot be placed directly with the correct figure but have to have the text edited after placement