Hi guys and Merry Christmas
I've just done a survey for an Architect of a house with a very expansive garden. The Architect is going to model the drawing in a 3d program and so has asked for some contour lines to be included to assist him with the modeling process.
The survey processing software I use (LSS - UK software) shows contours based on interpolated levels formed by triangles between surveyed levels.
In large areas the contour lines look great however in areas where there are retaining wall things become problematic.
For example I have a retaining wall where the base of the wall has a level of 60.00 metres. The top of wall level is 62.06 metres. I have a 61.00m contour running through the wall which makes it look like the level on the retained side of the wall is 61.00 metres where it is actually around 62.06 metres.
I haven't made any errors with the survey its just the way the contouring software interpolates the data. I'm worried that I could leave myself open to claims if the Architect thinks the level on the retained side of the wall is what the contour says it is.
What do others do when using contours on their drawings?
Kind regards, Andrew
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You need to draw the retaining wall as breaklines. What I do is to draw the outline of the bottom and top of the wall as 2 separate plines. Then in LSS, create a proximity breakline from these plines. Depending on how you shot the walls, the contours may avoid or go around the walls. Sometimes even after including breaklines, the contours?ÿ don't appear correct. I just manually edit the contours around the walls because I find it is faster this way.
You need to draw the retaining wall as breaklines. What I do is to draw the outline of the bottom and top of the wall as 2 separate plines. Then in LSS, create a proximity breakline from these plines.
Thanks for your reply jt1950. Could you expand on your comment please - I'd be very grateful.
Putting the contour lines in make me apprehensive because whether or not they represent realistically what is going on can be hit or miss dependant on the density of the spot levels that have been taken, the spacing of the levels and whether or not there are hard features are on the site.
I think a disclaimer in the drawing notes would be useful saying that, "contours are interpolated and their accuracy can not be relied upon. Where accurate level data is required spot level data should be used"
You could always add elevations at critical areas to define the structure(s) so there won't be any confusion come design time. Architects will blame the surveyor even if they misinterpret?ÿ elevations on a site plan. Cover yer butt!?ÿ ?????ÿ
For years I have shot the base of a retaining wall at the face, and the top at the back and created breaklines. The contour line within the wall should be hidden or trimmed within the wall linework. For highly detailed design topo a curb can be considered as retaining wall. Curb returns with small radii generally require some manual contour line manipulation to look right. I hope your architect knows that topo line precision is at +/- half a contour (height and contour to contour spacing). If that is not sufficiently accurate for his purpose he should pay for a scan.
Paul in PA.
Paul - what do you mean when you write about creating breaklines? Do you mean you just stop the contours at the retaining wall?
That is essential in order to have rational interpolation going on.
We have done many elevation certificate projects because the flood map has applied a linear assumption between contour lines that differ by 10 feet (typical USGS mapping).?ÿ There may be a bench area that jumps up something like eight feet that does not appear on the USGS map because it didn't get to the next major number for inclusion.
Think of something like a dam or levee.?ÿ The water level on the upstream side has nothing to do with the water level on the downstream side unless the depth exceeds the height of the dam/levee for a long period of time.
Does your "contouring" algorithm know how to adjust to match reality??ÿ Probably not, in may cases.?ÿ Thus, the need to stop and start again.
Paul is talking about creating a void in the surface model where things like retaining walls and building footprints are. His method is one way, and a good one. I use C3d, and previously LDD. There is a thing in those packages called "non-destructive breaklines" which uses the feature linework to create a breakline which creates that void, quick and easy.?ÿ?ÿ
A note of warning. It seems that you are unfamiliar with making DTMs. The collection of points and lines which form a proper DTM data set can be somewhat different from that you might collect for planimetric mapping with spot elevations.?ÿ
A breakline is an object you add to the surface which defines where the ground folds (like the crease in a folded piece of paper) or breaks. Unless you take 10,000 shots on a 100x100 plot the resultant contours may not reflect reality. The idea is you take fewer shots and take shots along features where the ground breaks (such as a swale or top of bank) then connect them with 3D lines so that the surface is created reflecting that continuous break.
More specifically a breakline is a 3D polyline from specific feature points to points. Using my example of a curb, you could sequentially shoot a 3D polyline along the bottom face of a curb in numeric order and come back at the top back of curb sequentially back. Then manually connect same in CAD, which was typical more than 30 years ago. Around 1990 cad software evolved with F2F, (field to finish), where the CAD program would automatically create the breakline sequentially for all BC and CC points. Most often used in creating an exisiting street profile, where you survey a street, say at 50' stations, from a grade shot 25' from ROW, @ rear of sidewalk, front of sidewalk, TBC, BFC, CL, BCF, TBC, FOS, BOS, G move 50' and redo. That would be 11 breaklines. In between the 50' crossections you would need, entry sidewalks an driveways, if the purpose was street reconstruction. If it was for sanitary sewer construction you would add exterior grade at house corners and a front door sill shot. Somehow all of this should be in your Survey 101 Textbook.
Contours crossing breaklines typically break at angles.
Paul in PA
Contours are an expression of the model (typically). The model of the surface is typically a TIN. This is seen as a series of triangles, each side of which we can call a "face". A "breakline" in practice will force the face of the triangle to follow the breakline, or rather inhibit a face from pulling through the breakline.
You can often manually "flip" the faces such that they do not cross the breakline, so it is not necessary to use a breakline to achieve the needed result. If you have a large project, using breaklines created in the field (field to finish) are a great tool.
The contours are extrapolated from the TIN (not directly from the measured points). You should perform some analysis to determine how well the contours match your shots (extra shots not included in the surface can be useful here), which would enable you to provide a note to the end user regarding the expected accuracy of your provided surface model. This is a critical point because if it is accurate to 1 meter, then the architect should not expect half-meter accuracy.
It is also best practices to include "spot shots" or elevations at critical points.
We struggle with simply providing CAD for any design. We typically include a digitally signed .pdf of the contours and notes and disclaimers, and include in our CAD files a disclaimer that the signed survey is the only product that we certify. CAD files can be changed and there is a real possibility that they will be misused.
Somehow all of this should be in your Survey 101 Textbook.
What you learn from textbooks and actual survey are 2 different animals.?ÿ
