Survey elevation (relative to sea level) and project elevation

[chughes]

How can I create a toposurface whose elevation points are relative to sea level and not relative to the project elevation?

I have moved my project elevation to where top of slab = elevation above sea level.  I leave my survey elevation at 0'-0" (sea level).  I now want to draw a toposurface based on a survey with elevations relative to sea level.  However, when I draw my toposurface and input my point elevations, the point elevations are relative to the project elevation, not the survey elevation of 0'-0".

For example:

Project elevation = 15'-0"

Survey elevation = 0'-0"

Toposurface point elevation = 14'-0" (I would think this would put my grade 12" below my top of slab)

My toposurface is 14'-0" above my project elevation (top of slab) and 29'-0" above the survey elevation (sea level).

See below for reference.

What am I doing wrong?

Thanks.

[Sahay_R]

Is your level Elevation Base set to Project Base Point or Survey Base Point? I would think that the latter would solve the problem.

[chughes]

Level elevation is set to Project Base Point (top of slab).  I wouldn’t want my project levels to reference Survey Base Point (sea level), only the toposurface (and possibly a few other site conditions).

[Sahay_R]

Model the Toposurface into its own file, link the architectural model. 

[chughes]

Ideally, I would want to avoid creating and linking a new project.  Is having the toposurface reference the survey elevation information and not the project elevation information within a single model not possible?

[barthbradley]

Just draw your toposurface and move it down 15 feet.  Spot Elevations can be relative to PBB or SP.  

[ToanDN]

You can gave different Spot Elevation types.

[chughes]

I understand that spot elevations can relative.  I also know that I can draw it and move it vertically.  If I want to come later and manipulate the grade (add driveways, walkways, landscape features) I cannot work in native elevations if I move it vertically.  It sounds like the answer is 'No, you cannot have the toposurface reference survey elevations.' and future grade changes will be from the project base point.

[chughes]

The below example exhibits the difference.  The preference is the defined elevation of the point on the toposurface is the same elevation as the contour line elevation.

[barthbradley]

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@Anonymous wrote:

 If I want to come later and manipulate the grade (add driveways, walkways, landscape features) I cannot work in native elevations if I move it vertically.  

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Not following you on this statement: "cannot work in native elevations".  What is the "native" elevation?  But, if you want a different workflow that achieves basically the same thing: move all your toposurface points down while in sketch mode and then hit finish. 

[barthbradley]

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@Anonymous wrote:

The below example exhibits the difference.  The preference is the defined elevation of the point on the toposurface is the same elevation as the contour line elevation.

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You're highlighting a toposurface point (-1'-0"). How is this germane? If you close out of editing mode and place a spot elevation on it, it can read that "spot" (where the point is located) relative to Project Base Point OR Survey Point. 

[barthbradley]

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@Anonymous wrote:

....you cannot have the toposurface reference survey elevations.' and future grade changes will be from the project base point.

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No, that is an incorrect statement. 

[chughes]

It is germane when you are defining locations for the 16' elevation contour to follow.

The contour line elevation relative to the top of slab is less significant than the contour line relative to surveyed back of street curb or site wall.  The toposurface point elevation is what controls the direction and flow of the contour lines.  The location of my slab (and Project Base Point) is controlled by site conditions, not the other way around.

I am simply looking for a way to have the toposurface points reference actual true elevations, not Project Base Point.  It would seem that I could draw the toposurface to true elevations, based on the Survey Point, without having to draw it first and then move it on the Z axis.

[chughes]

Native elevation, in this instance, is 16'-0" NAVD.

To make future manipulations to the site, after I draw it and move it, I create new toposurface points at -1'-0", not 16'-0".

[SteveKStafford · Accepted solution]

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@Anonymous wrote:

Is having the toposurface reference the survey elevation information and not the project elevation information within a single model not possible?

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Toposurface points only reference the Project Coordinate System (PCS). If you create a Toposurface based on an external source the resulting points and surface reference it's position relative to the PCS.

I always use a separate model for site/surface conditions so the surface can be at actual site elevations. I link the building model(s) to site and move it up to the intended ground floor elevation. This allows me to show any elevation conditions I need to, either arbitrary values at 0' or 100' or actual sea level related values; via spot elevations and level annotation. Combined with survey data this allows me to manage the overall location data for a building or collection of buildings effectively too.

Sounds like you feel it is a bit much to use a separate file just so you can enter point values for a surface using values that match the survey. As such, you could...

• create the toposurface using the actual elevations (it's too high now)
• move it down to the correct elevation
• To edit it later, move the toposurface back up to the actual elevation
• Edit/add points - Finish
• move it back down to the relevant elevation

For example, a toposurface that sits at 215' above sea level you'd move it down 215' after creating it, then move it back up 215' to edit/add points, and move it back down the same 215' when finished. Just the extra move up/down versus working with a separate file.

[chughes]

All, thanks for the feedback.  I do feel it is too much to create separate projects for site and building, especially on single family projects. 

I will move the toposurface vertically as needed when I need to manipulate it.  Sure would be nice to set it and forget it, though.

[SteveKStafford]

Felt the same way at first, but over the years I've learned to just use a separate site model. I find it easier to resolve requests and issues that arise during the course of a project. For example, when someone decides the building needs to move elsewhere on the site, or they break it into separate elements, like a detached garage or in-law apartment...

[Sahay_R]

I'm in the same camp as @SteveKStafford. These are two entirely different types of information. Divide and rule is the name of the game.

[jfrazierdorsky]

I know exactly what you are wanting to do, I don't think these others do. Did you ever solve it? And not have to link a model in? I just want my site elements to be based on actual survey numbers and my building to still have its ground floor at 0'-0"  Seems like it should be possible given that they give you both a survey point and a project base point.  But you can set the topo surface to align with one or the other, which sucks, then you have to do math which I hate.

[barthbradley]

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@jfrazierdorsky wrote:

I know exactly what you are wanting to do, I don't think these others do. Did you ever solve it? And not have to link a model in? I just want my site elements to be based on actual survey numbers and my building to still have its ground floor at 0'-0"  Seems like it should be possible given that they give you both a survey point and a project base point.  But you can set the topo surface to align with one or the other, which sucks, then you have to do math which I hate.

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Huh?  

You say:

I just want my site elements to be based on actual survey numbers and my building to still have its ground floor at 0'-0"

What's stopping you?