Surface from GIS data-vertical datum

[klugb]

I am using the import surface data from GIS data and noticed that it is converting the vertical datum or elevations of the contours. I have seen other posts about Civil 3D not having a vertical datum setting, but there must be something hidden because the contours in GIS are NAVD88  but the imported elevations match our old datum of NGVD29(47), approx 3.3' lower.

Our horizontal datum is "HPGN (HARN) Washington State Planes, South Zone, US Foot", but I can't see any connection to the vertical datum.

Any help would be appreciated.

[Joe-Bouza]

is this page of the wizard gumming it up?

[ChicagoLooper]

The command you're using, CreateSurfaceFromGISData command is limited, as evidenced by the variations in topographic contours in your uploaded images. Ideally, your imported contours should match, fall directly on top of, the GIS contours.

A more sophisticated workflow would be:

1. Using a brand new, clean template, run mapimport command (with 'Create Object Data') to import the contour shapefile.
2. In new drawing, attach the dwg file containing the imported contours. Run a query. Combine this query with 'alter properties' to raise contours from 0 elevation to their proper elevation.
3. Create a surface using the properly elevated contours.

Can you upload you data?

shp, shx, dbf and prj.

[Pointdump]

Bruce,
Interesting. I've never known C3D to care, one way or another, what vertical datum is assigned to your surface, or the data that makes up your surface. No vertical datum conversions.
Can you hit us with a link to your original data and the project location?
Have you done any static GPS in your area of interest. Then you'd have ellipsoid height which could be converted to NAVD88 with either Corpscon or >>>NCAT<<<, as a check on your data.
Dave

[klugb]

Joe, I don't think there is any connection between the horizontal datum and the vertical datum.

ChicagoLooper, The contours converted to the correct datum ( I usually have to manually adjust the 3.3' shift between datum's) The problem is I can't figure out how Civil 3D new to convert the data between datum's. The file is are too large, it's a county-wide contour shape file that's 922 Mb's.

PointDump, The file is are too large, it's a county-wide contour shape file that's 922 Mb's. Technically, the data is correct in both locations. In GIS they are at the NAVD88 elevations and in Civil 3D after the import they are at the NGVD29 datum. I just can't figure out how Civil 3D knew to convert during the import. I have ground survey data in the NGVD29 vertical datum that matches (closely) to the imported GIS contours.

[Pointdump]

Bruce,
Is your GIS data publicly available? What county?
If you've uncovered some hidden bug in C3D I want in on the ground floor.
Dave

[Pointdump]

Bruce,
OK, I'm going to guess Clark County and start searching.
Expert Elite "Alumni"? What's that?
Dave

[klugb]

Yes, you can access the public data, but not sure you can download the contour data.

https://gis.clark.wa.gov/mapsonline/?site=Environmental&ext=1

[klugb]

I put a link in my other post.

I was one of the original Expert Elite's but haven't been active for a couple years so Katinka had my profile switched to "Alumni"😅

[Pointdump]

Bruce,
"...haven't been active for a couple years..."
I guess I better keep my game stepped up so that Katinka doesn't "Alumni" me. 

Going to your GIS site now.
Dave

[Pointdump]

Bruce,
I can go right to it, but I think I've got to "sign in" to download. I can't figure out how to create an account.

So what is a "datmun"?

Dave

[ChicagoLooper]

That Clark County site with open data is confusing.

Anyone can download public data, no problem. Before downloading, you can view the metadata for contours. Here's the metadata for two different shapefiles (you view yourself using the url shown in  blue):

The site lets you download the data.

Which data is downloadable? Retired? Current? Or both?

Go here:

Maps & Records=>Public Records=>Open Data Hub=>Digital GIS Data Bulk Download=>then download (a) Contents of Volume 1 and (b) Contents of Volume 2.

You can't download individual shapefiles, the data is prepackaged in two volumes.

[Pointdump]

@ChicagoLooper,
Wondering same as you. I downloaded "Volume 2" and there is a "Contours.shp" of size 992MB. I used MAPIMPORT to bring that monster into a drawing. Took forever. Now I'm using >>>MAPTRIM<<< to clip those contours to Bruce's area of interest. C3D is crunching. I hope it doesn't crash.

Dave

[ChicagoLooper]

@Pointdump 

Instead of bringing in that monster shapefile, just bring in a small portion of it. You can define your area by drawing a regular, garden variety rectangle. Then instead of clicking 'Add to Map' click Add to Map with Query. 

 

Your query will go like this:

Locate on Map=>click Polygon from the dropdown=>then on command line click the 'S' for select option=>finally, using your square cursor, click your garden variety rectangle.

If you want to delete or redefine your 'area' you can do this:

Right click contour layer in Map Task Pane=>select Query to Filter Data=>Highlight the query (the blue text) and delete it. 

[Pointdump]

@ChicagoLooper,
That's a good way, but to duplicate Bruce's situation we need those contours at elevation. So MAPIMPORT is necessary. And anyway, I wanted to try out MAPTRIM. Still working on this. Civil 3D is choking on that huge shapefile.
Dave

[klugb]

Our GIS department recently switch from one datum to another. Our Survey dept still uses the "old" datum.

When I import the contours from the SHP file using the "MAPIMPORT" command and then populate the contours (polylines) with the elevation data field it matches our GIS online contours. With this method you would have to manually convert the elevations to the other datum, but using the method I described earlier it automatically converted. BUT HOW DID IT KNOW?????

[ChicagoLooper]

The MAPIMPORT command also allows you to create window for your area of interest.

I used a window for MAPIMPORT when bringing in Clark County Data. I also used MAPIMPORT without a window to bring in another shapefile from The National Map. The TNM has downloadable contours too (Topo Map Data and Topo Stylesheet) in either shapefile or GBD format. 

Creating two surfaces, one from Clark County contours and a second from TNM shapefile contours will allow a comparison between the two datasets. In the image below, green & red TINN contours from Clark County data. Gray TINN contours from TNM data. A-A' and B-B' intersect adjacent to the intersection of NE 119th St and NE 152nd Ave.

A surface profile adjacent to 119th St. (A-A') and another (B-B') adjacent to 152nd Ave. is an easy way to compare the resulting surfaces.

 

The vertical exaggeration is 20.

Blue=Profile created from Clark County contours.

Red=Profile created from The National Map (TNM) contours.

The blue profile has a lower elevation than the red profile which may, or may not, be due to the vertical datum. The red profile generated from TNM data appears to be smoother, way smoother.

[ChicagoLooper]

Based on what you've posted, your GIS department and Survey department use different datums, NAV88 and NGVD29 respectively. Correct? If correct, then why?

If the above is correct, then you have two options:

1. Two different shapefiles, with each one using a separate dataset. You can't expect Civil3D to perform the conversion from NGVD29 to NAV88. 
2. One shapefile with two separate elevation fields: one field would be NAV88 and the second would be NGVD29. With two distinct fields, you could build a surface by choosing the elevation field of your choice.

[klugb]

Yes, they do.

[Pointdump]

Bruce,
I'm not seeing what you're seeing. I created, and have attached, a subset of that monster shapefile. Using that, I Drag-n-Dropped(same as MAPCONNECT) it into a new empty drawing, and styled it thusly. It does not match your "Raw Contour Data in GIS". More matches your "Imported using Create Surface from GIS Data" pic.

Dave