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.
(They’re all inferior to Civil 3D, aren’t 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 surveyor’s 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/<surface name>’ under the project name. In Civil 3D,
the easiest way to import this type of data is to create a
new surface using the “Create Surface from TIN…” choice from
the Prospector tab. Creating surfaces with DEM works
similarly but DEM is usually reserved for very large
surfaces and the data can sometimes produce surfaces that
look “boxy.”
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. Don’t 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.
