Often times, users are forced to create surfaces with what most would consider less-than-optimal data. For example, the owner of one of your projects commissioned a survey prior to your involvement and the survey was pieced together with older or inferior software. (Theyre all inferior to Civil 3D, arent they?) The owner proudly informs you that the survey has been done by a friend of his and he will email it right over to you. Lucky you. It is now your task to take that data and make lemonade from lemons.
Elevation data can arrive at our desktops in a wide variety of formats. Sometimes we have the luxury of choosing which format to use, sometimes we get stuck with contours. In any event, we must make a decision on what is best to use for creating surfaces in Civil 3D and execute that task to the best of our ability, both preserving the intent of the surveyor and creating an object usable for our Civil 3D tasks. Here is a list of all the different data sources presented in order of best to worst with some pros and cons for each.
1. XML Data This non-proprietary data exchange format is used to store and share a variety of different object data types through an industry-specific code called LandXML. When data arrives in this format, we know the surveyor was working with current software and that the integrity of the surface as he or she created it will not be compromised upon import to Civil 3D. The triangulation which originally created that surface in the surveyors software remains intact. This is the preferred method of creating a surface in Civil 3D with data from an alternate source because it is easy to import, it retains its original shape and lines, and it is accepted by many software brands in the event it needs to travel beyond your desktop. As a Civil 3D user, you may have to make a special request of the surveyor to export his surface in this format, if his software supports it. There are no real downsides to using XML data to create surfaces in Civil 3D.
2. TIN File Much like XML data, a TIN file is built using the triangulation lines that were originally used to define a surface. This is another excellent way to build a surface in Civil 3D. Many surveyors still use older versions of Land Desktop that do not support LandXML and if that is the case, request the TIN file which he can find in the subdirectory DTM/
3. TIN Lines The triangulated lines that make up a TIN are fine to use for creating surfaces in Civil 3D. These lines are often found in a layer from Land Desktop drawings called SRF-View. The difference between using these lines and the actual TIN file is that to use the lines, we have to go through an extra step or two. Also, when the TIN lines are used, they are added to the definition of the surface we must create as Drawing Objects and are now subject to their own triangulation, meaning the software may draw extraneous TIN lines outside the limits of our actual survey. So there will often be some cleanup associated with this method. In addition, in order to add our own elements (breaklines) to these surfaces, we must make sure to keep the Maintain Edges from Objects option unchecked when we choose the actual lines from the drawing.
4. Contour Data This method is not preferred but sometimes we have no other options. In this case, a surveyor may have sent us a DWG file with just the contours to represent elevation data. This is an unreliable source because it tells us nothing about the elevation information between the contour lines. Also, despite recent Civil 3D improvements, this method can create multiple flat spots on our surface because the contours themselves do not retain any sort of information on breaklines, ridges, or draws.
5. Points This is a reliable way to produce surface data but, if we did not perform the survey itself, it forces us to make some assumptions about what data was intended for topographic use and what data is unsuited for inclusion into a surface. For example, if the surveyor shot a fire hydrant, did the rodman set the prism rod next to the hydrant on the ground or did he set it on one of the bolts? How do the flowlines and ditch banks fit together? How do the building corners fit together? What do the different description codes mean? It is always best for the surveyor who shot the data to piece together the topographic map and existing ground surface.
Regardless of the source or format of the data, it is always advisable to maintain that original file as it was sent to you. Always create and work with a copy of that data. When something goes wrong with a job and fingers get pointed, you always want to have an original copy of that data for reference.
Making smart decisions about the use of data from outside your organization is what being an engineer or designer is all about. Consider your sources, analyze the alternatives, and proceed with some caution. Dont be afraid to ask for data in a different format, especially if you are unsure about some components of the surface given to you. Always remember that the software is still just a tool in the overall realm of good judgment and sound engineering.
http://newsletters.hagerman.com/newsletters/ebul65-CIVIL.htm
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