Terrain Elevation Discrepancies

[javier.barcelo]

Hi there,

I am having trouble to understand why I have such massive differences in terrain elevations. This is my workflow:

1. Model created from model builder including all features available (terrain, imagery, roads, etc.)
2. TIN (NS) created in Civil 3D from survey 3D faces. File imported in Infraworks and configured to be in the same coordinate system
3. TIN (NS2) created from asc file. This file comes from SRTM-derived 1 second DEM.
4. Import into infraworks NS & NS2

When comparing the 3 different terrains I get the following, using NS as my datum

• NS = 0m
• NS2 = +2m (approx)
• Infraworks terrain = +5m (approx)

Does anyone found such discrepancies? I understand the last two terrains cannot be as accurate as a proper surface but a 5m error!! FYI, this model is in an industrial environment nearby Sydney's airport, Australia. It's a pretty flat area without big hills or dramatic elevation changes.

I am trying to "sell" this software to my company but if I cannot provide a higher level of accuracy there's not a chance it'll pass the approval test from management.

Regards

Javier

[Karsten.Saenger]

Hello @javier.barcelo

this question is more a question of how accurate is the SRTM data in different grid sizes.

In the model builder you can read the following:

So, you are comparing SRTM 30m grid data to SRTM 1 arc second grid data that you have. Actually the 1arc is the same as 30 meters, but there are different SRTM versions available. Depending on what version you have it could be much more accurate compared to the data that the Model Builder is accessing from a server.

See also: https://earthdata.nasa.gov/nasa-shuttle-radar-topography-mission-srtm-version-3-0-global-1-arc-secon...

Regards,

Karsten.

[Glenn.Coppard]

Have you checked that the vertical height datum is compatible between the data sets?

As there is no place to specify a vertical height datum in IW360, you will need to determine this based on your most accurate dataset.

eg. Your survey could be using AHD or something site/project specific, where as the SRTM may be off some geodetic datum or spheroidal height, and as Karsten mentioned, there are different version of SRTM that have different elevations.

You will need to find out the 'difference' between your survey datum and the above to understand fully why there are the elevation differences you've quoted.

Also note, that comparing SRTM to 'survey' quality data is not a measure of accuracy, they are used for completely different purposes, and you projects should be built up with this in mind.

We would typically use SRTM give spatial context of a scheme, LiDAR / DEM's give the detail around the immediate periphery of the design content, and where critical interactions occur, we'd have site survey to integrate the design onto.  And the 'Terrain' within IW360 is built-up accordingly.

[javier.barcelo]

Thank you for your replies Glen and Karsten. I understand what you guys mean. The problem I have is that a model created in C3D, an access to a private property from a median-separated two-lane each way road, a  couple of slip lanes, islands. etc. Nothing overly complicated. I would like to bring my finished TIN into Infraworks for presentation purposes. If I import my finished terrain into Infraworks that, remember, is 5m lower than the terrain that came from model builder, well, the result is less than desirable. It's actually useless.  I tried bringing the survey TIN too, as it covers a larger extent than my design TIN, but wherever the survey finishes it creates a massive batter to climb up 5m to the model builder terrain.

So, with all that in mind. Does anybody have any suggestion about how to integrate your design model using survey levels into Infraworks interfacing nicely with the model builder terrain?

Cheers

Javier

[Karsten.Saenger · Accepted solution]

Hi @javier.barcelo

thanks for the explanation.

So, the main question in the moment is how to merge all surfaces in InfraWorks so that it looks nicely.

With the current dataset you have it is not possible to do that in InfraWorks, only option is to export the current SRTM surface from InfraWorks as IMX and reimport it with a vertical offset.

Better solution - and more control - is to go via Civil 3D. Export the SRTM-Model Builder data as IMX. Bring it into Civil 3D and raise/lower the surface, create grading, edit triangles, until it best fits to your reference surface.

Then export as IMX and import it into InfraWorks again.

Regards,

Karsten.

[roskirko · Accepted solution]

The elevations initially used by Infraworks are based on the SRTM program (Space shuttle Radar Topography Mission) data freely available from agencies such as the USGS, for example (see their Earth Explorer interface online and look under the Elevation Data sets). The elevations from the SRTM are averaged over an area as small as 10 meters square (in the contiguous US) but up to 30 meters square elsewhere and are therefore prone to error.

I used photogrammetric techniques (aerial stereo photography) using PCI Geomatica as well as using LIDAR data from the LAZ.zip organization, which is free, from http://www.laszip.org/ 

The LIDAR data can have the elevations created using Infraworks's Point Cloud processing tools (https://knowledge.autodesk.com/support/infraworks-360/learn-explore/caas/CloudHelp/cloudhelp/ENU/Inf...).

LIADR (point cloud) data has greater precision than the SRTM data but the photogrammetric techniques are even better.

Finally, Ground surveying data using GPS to gather elevations from which Civil 3D surfaces can be created and turned in to GeoTiffs for use as an elevation raster data source is the most precise data source.