Topographic surveys are concerned with "terrain" -- the lay of the land you might say. Having accurate site data allows the site designer to tailor contemplated improvements to what already exists, and may allow the construction to cost less by using earth taken from one part of the site to fill another part, etc. It is essential to good site design. After all, a contractor at the start of work encounters the site as-is. If the design also began with that same site "picture," the resulting instructions (i.e., the construction plans) should "fit" the site like a glove.
Things to consider
There are a couple of considerations when ordering topography. Obviously the limits of the survey need to be agreed upon, but the orientation of the survey could also be important. By orientation we mean whether the survey is referenced to a city-wide or state-wide coordinate system* or referenced to an established height above sea level. If the proposed improvements will require tying into existing public utility lines (water and sewer), the reviewing agencies will likely require that the design be on the coordinate system used by that jurisdiction. Conversely, projects that lie outside of built-up areas and that will have no utility impacts might be candidates for topography on an "assumed system." In those cases, we assume an elevation (height above sea level) at some spot, and reference the rest of the work to it. In the same fashion, the survey might not be oriented to true north, but would "assume" a north in some direction, and proceed from there. As long as the design focuses only on the site itself, assumed topography supplies the designer with an adequate picture of site conditions. And, it saves the cost of tying the work into the larger "system."
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The identification and location of underground utilities could be important. The site designer will be very interested to know (to the extent possible) the nature and location of any underground pipes or other service lines. Sometimes we are asked to inquire of the local government about utility construction plans that may have been deposited with them. (The plans for underground utilities generally provide the most complete understanding available of them--other than mass excavation, which is very disruptive and expensive!) Other times designers prefer to make those inquiries themselves. Either way, an understanding of site conditions both above and below the surface of the ground is important to good site design.
If the area of interest is in the vicinity of a property boundary, or of a zoning setback line from a property boundary, it could be that determining that boundary location will be necessary so the designer can place the proposed improvements appropriately.
*A "coordinate system" is an imaginary grid laid over the region, oriented North-South and East-West, that provides a convenient "spatial reference." Locations specified on such systems have a North value (a Northing) and an East value (an Easting), which are distances measured from the "axes" or lines running due north and due east from the system origin to the point in question. Remember your high school analytic geometry?! Usually coordinate systems are implemented by local or state authorities and used by them--and by extension, others they require to use them--for all matters requiring a precise location definition. In the Baltimore area there are a number of active coordinate systems in use. See how they interact with one another on the New Martenet Atlas page. As you move the cursor around the screen, the coordinates in five coordinate systems update as well. And onscreen is the representation of one of the systems (called "the Maryland Coordinate System NAD27") which appears like a checkerboard everywhere but within Baltimore City.