Topographic Surveys - Part 1

As mentioned in my last post, there are times when a client needs only topographic and planimetric information. Instances where this may be the case are projects that only require filling of earthwork, construction of a sidewalk or something similar.

Many times we get requests for topographic surveys that show aerial photography and a big, red circle that says "Survey Limits". While this may seem like all the information needed for the surveyor, I think it's only a very small part of the information we need. The first thing I always ask our client is always the same question: "What's the real purpose of this survey?" Certainly, the answer to this question allows me to have a better understanding of what's really important inside the magical red circle. It also allows me to determine the level of accuracy needed to accomplish the task.

Regarding the vertical datum or basis of elevations for the topography, we always need to know who is going to use the survey. For instance, in some cases it may be adequate to assume an elevation of 100.00 at a particular location. However, if we know that it may be near a flood plain or flood fringe, we may need to use National Geodetic Vertical Datum (NGVD) 1929 as many of the Flood Insurance Rate Maps are based on this datum. Other times, a particular governmental jurisdiction may have a Geographic Information System (GIS) based on another datum such as the North American Vertical Datum (NAVD) 1988 or even possibly on a long established assumed datum.

Similarly, we need to know what horizontal datum or coordinate system the survey needs to be on. Many times our clients have heard of State Plane Coordinates and instantly reply that they prefer that system. However, you need to be warned that state plane coordinates, universal transverse mercator (UTM) coordinates and many other coordinate systems covering large areas are not a one to one conversion to coordinates that can be established on the ground. I'll get into that more in another post as it needs to be a complete post of its own.

Often we see that our clients want a temporary bench mark set on the construction site. My opinion is that having only one bench mark set on the site is a bad idea. If the elevation shown on the plans is shown in error (let's say its 700.91 but inadvertently shown in the plans as 700.19) it could create some major problems with the intended construction of the project. We have a company policy that we never set less than three bench marks on a site so that we always have one that we assume is good and one to check it with in case one is destroyed prior to construction. For larger areas we normally recommend to our clients that we set one bench mark for every six and a half acres. Why six and a half you ask? The optics on a typical automatic level that mights be used by a contractor has a usable range of about 300 feet (and it's about as good as our eyes are). The area of a circle with a radius of 300 feet is about six and a half acres.

Another piece of information needed by the surveyor is the contour interval to be shown on the drawing. Many times the purpose of the survey leads us to the determination of the contour interval required as the more accuracy required, the smaller the contour interval is normally going to be. There are many times that specifications for surveys indicate that not more than 10% of the elevations cannot exceed an error of more than one-half the contour interval, a specification that has been perpetuated since the National Map Accuracy Standards were first developed in 1941 by several government agencies including the U.S. Geological Survey (USGS).

In the next post, I'll talk about what other information is needed for topographic surveys after the basis of data is established.