Comparing toposurface generation: Lidar versus CAD contours

[luisPDAN6]

A came across this article that compares the differences of generating Revit toposurfaces from CAD contours versus using Lidar point clouds. I was quite surprised that the generation of terrain using cad contours significantly poorer than the terrain generated from the point cloud. Are there many users here at the forum using Lidar point clouds? Our surveyor is doing drone scanning of terrains and we are going to order a Lidar survey next time.

[Pshupe]

How surprised are you that a company selling a service to import LiDAR data suggests that this type of import is much more accurate?  😉. Is that accuracy required?  Most times no. Is it worth the memory to incorporate that amount of data?  Again most times no.  Also it is not saying that contour lines are inaccurate but that LiDAR is capturing much more detail.  Most projects would be surveyed, so LiDAR would be an additional cost, not to mention the cost of this import software. Revit can automatically create terrain from contour lines without the need of a third party app. I’ve worked for Architects for over 20 yrs, most with Revit, and I have never needed more detail than what a 1m contour survey supplied. There may be some projects that would require, or would be nice to have, more detailed info but I have never worked on one. 

Regards Peter. 

[pablo_kemp_1988]

The surveyors we use already employ a Lidar drone regardless, because it saves him time. Labor costs are high over here, so it makes sense to invest in good equipment. I personally prefer accurate surveys than errors later on, because buildings are not that cheap and we think we deliver a better job. I understand that in other parts of the world the transition to Lidar surveys may be a bit delayed due to lower labor cost and relatively higher cost to buy equipment. What we also get for free is access to a national Lidar database, and we can download whatever site we want. That is far better than buying contours if you ask me. Paying for less precise information makes no sense.

[Anton.Czerwinski.1965]

I think that the article manages quite well to prove its point. Revit terrain deviations in the magnitude of 0,5m by using contours is not acceptable. Buying a tool for importing Lidar points to Revit has a cost, but it gives good results. 

[Pshupe]

I work for a company that provides Lidar services for surveying and Civil applications as well as as-built conditions for Architects and Engineers for construction documents.  As I have mentioned, I have worked for Architects and Engineers for about 25 yrs creating construction documents.  Most Architects are more than happy with 0.5m contour lines for their terrain modeling.  I just do not see the advantage for more detail.  More detail = slower machine.  Similarly, why we do not 3d model everything.  It can cost you more money for Lidar and then slow down your construction documents.  Most of our civil / survey consultants also use Lidar but give much simpler contour information right out of Civil3d in dwg format, which is easily imported into Revit, for free, with the perfect amount of information.   The topography changes from existing conditions to as-built anyway and there are always excavations for basements and footings etc.  0.5m is lots close enough IMO.

Regards Peter.

[lucdoucet_msdl]

@luisPDAN6 

I agree with @Pshupe that the level of detail of the lidar scan is not always required nor useful in Architectural work. 

As an architect working at all phases of the building process, I want to work with the data that is relevant to a projects specificities. Analyzing a site for a prospective project in a green field, 0.5m contours are fine as opposed to an urban site with property line elevations that will decide the interior floor elevations and building configuration, I may want a Lidar Scan.

When passing from schematic design to working drawings, neither the contours or Lidar will do as I need a survey to report certain features to 1cm precision which I may have trouble identifying from Lidar data.

Also contours generated from human survey data may incorporate geographic features such as valleys, peaks and  berms that Lidar data sets may not interpret correctly due to the use of the algorithms used to separate the ground from vertical features.

Hope this helps,

-luc

[lucdoucet_msdl]

@luisPDAN6 

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Are there many users here at the forum using Lidar point clouds? 

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To answer you specifically, I use the AEC collection of tools with public Lidar data for building site context and interior to the property lines when the client hasn't supplied the topographic survey of the property. This means a combination of Autocad, Civil3d, Infraworks, Recap and Revit.

When passing to detailed design, interior of the property lines and a @3m perimeter of the topographic data will be updated to what survey information has been provided by the clients surveyor. 

I prefer generating the topography from Lidar data in Infraworks rather than Civil 3d until I go to detailed design where Civil 3D more flexibility in merging and massaging the various data sets.

-luc

[luisPDAN6]

For what I can read, in the process of generating terrain directly in Revit the deviation is quite considerable between 1m contours and Lidar. 

[luisPDAN6]

@lucdoucet_msdl , I pretty much only work in urban areas where relation to existing roads, sidewalks and buildings are critical. Lidar is a good choice for surveying the heights. For boundary lines we use official information of specific road/street projects in CAD format.

An airborne Lidar survey has theoretically a 2-3 cm max deviation depending on the sensor. That is a fairly good starting point in terms of accuracy and should generally avoid major changes of floor levels, redoing stairs, and avoid significant revisions for the other consultants at later stage (detail design for instance).

Lidar clouds can be made thinner to avoid taking too many points into the Revit topography. In that way it is possible to get a well balanced net of points and prevent over flooding Revit with information. It is also possible to have two levels of detail, one for the site and another for the surroundings. 

[Pshupe]

It may also be worth noting that anything that is of importance, can have an exact point placed in the survey, whether manual or Lidar.  The important thing for me is keeping the information light and not bogging down the project file with data that is of little use.  If you need exact measurements in a certain area then just take more points in that area.  I don't need to have the 1 acre site with 2 -3 cm maximum deviation over it's entirety and have the file size unruly.  I've been on both sides of this.  Working for the Architect and understanding the data that is required and then also trying to sell Lidar scanning to Architects for site topography or scan to BIM for the as-built buildings.  IMO most of the time neither requires such a fine level of accuracy, especially considering the memory issues that can crop up with such a dense topography model.

Regards Peter.

PS - on those very few projects where it is necessary high resolution Lidar is quite handy but it is surely not the norm.

[luisPDAN6]

I agree that it is very important that the topography element is not too large, or with too many points with the current way Revit works. So when we use Lidar as a source for topography, we thin the Lidar point cloud and we stay under a reasonable amount of points.

[ToanDN]

Isn't it called garbage-in garbage out?  If the contour lines in CAD are smaller interval instead of 50 cm apart then Revit can generate more detailed topography, or can't it?  Also, if the contours lines in CAD are exported from Civil3D then it might be better to export to a point file instead of a DWG.

[lucdoucet_msdl]

@luisPDAN6 

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An airborne Lidar survey has theoretically a 2-3 cm max deviation depending on the sensor. That is a fairly good starting point in terms of accuracy and should generally avoid major changes of floor levels, redoing stairs, and avoid significant revisions for the other consultants at later stage (detail design for instance).

 I think you may be citing the precision of ground based lidar measurements versus airborne surveys. My experience is that a lidar survey from a series of points on the ground or from a roof will have 2-3cm precision from a fixed position. As a comparison, the publically available aerial lidar data from a moving plane (trains or automobiles 😁) in my area will be of the order de 20cm. You get what your tax dollars pay for!

See the PDF attached and extract below. 

altimetrie = height dimensions and planimétrie = plan dimensions

As for a drone survey, I suppose there are high end models equipped with 2-3cm precision lidar.

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Lidar clouds can be made thinner to avoid taking too many points into the Revit topography. In that way it is possible to get a well balanced net of points and prevent over flooding Revit with information. It is also possible to have two levels of detail, one for the site and another for the surroundings. 

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Yes, this is exactly what I do using Recap in the AEC Collection. I will isolate the Lidar points within the sights from without and save them to two distinct lidar files. The Decimate tool in Recap will then remove a percentage of points from the originally dense data set to whatever density I feel is enough (ex. an 80% reduction of points @ 20cm spacing will give a 1m spacing of points).

-luc

[lucdoucet_msdl]

@luisPDAN6 

I agree with @ToanDN in that you need to compare apples with apples.

If I want to judge the accuracy of a Revit toposurface from 1m contour file against a lidar file, the lidar point spacing should be similarly spaced. If the lidar file has a point density of 30cm x 30cm spacing in plan, then compare it to a surface generated by contour spacings at 30cm intervals produce by a more precis surface model.

The selling point of the blog posting is that they are implying that a manual surveyor will generate contour lines using less points than the lidar equipment and/or commercially available contour maps are not as precise as commercially available lidar data sets, therefor the resulting toposurface from a contour map will have less detail. Duh!

This said, if you have the tools to process lidar files (and there are free tools), they will probably be more precise, more recent and more adaptable than contour files, be they publically available or generated by a surveyor. 

-luc

[Anton.Czerwinski.1965]

@ToanDN What is the distance of the curves publicly available in your country? In most European countries the distance is 1 meter. And for many european countries Lidar is also available for free. So it makes sense to compare both sources that do not require a specific paid survey in my opinion. With Lidar we can go to civil 3D and generate tighter curves, but we cannot generate more detailed elevation maps using just 1 meter contours. 

[luisPDAN6]

The 20cm deviation that you refer to is about QL3. I believe many countries are at QL1 with 10cm max deviation vertically and a bit more horizontally.

When I was talking about 2-3 cm, I was thinking about drone surveying sensors, that fly lower and slower. And they are getting more and more precise. Have a look.

For sure 30cm curves help reducing that deviation. But producing such curves without Lidar is really big dollars!

[luisPDAN6]

@ToanDN 

Why do you say it is better to use points from Civil 3D and not CAD contours?