What a Datum, or Benchmark Is
Working with measurements, it is extremely important to exercise control. If you are not exercising control, then things don’t generally come together properly. One way to exercise control with vertical measurements is to work with datums and to establish a primary benchmark datum that gives a localized origin to a design or construction project’s system of vertical measurements. By the way, the plural of datum is technically data, but my experience (and my research) suggests that in this context, saying “datums” is customary.
A datum is literally a data point, defined basically as any measurement or result. As a term it has many uses, but in design and construction, a datum refers to a vertical measurement taken in relation to a universal elevation, such as sea level.
Some folks use the word benchmark interchangeably with the word datum. It is true that a benchmark is a datum in and of itself. But in common use of the term, it is not just any datum – in a system of vertical measurements, a benchmark is a special datum. In complex operations, there may be a system of benchmarks called out, but in a smaller operation a single benchmark will often be isolated as the “ground zero“ of the system, the primary plane of reference, the sun in the solar system, the queen of the hive.
How is a Benchmark Datum Established?
A benchmark datum can be established quite arbitrarily by simply driving a sturdy pin (a.k.a. nail) into something of relative permanence, be it pavement or a tree or a masonry wall or any substantial thing that is immobile. The head of that pin becomes the benchmark from which all vertical measurements are taken; using measuring devices like a transit or laser level, that pin becomes the ultimate point of control in a vertical system of measurements.
When it comes to measuring, error tends to develop in accumulated measurements. For instance, the fractions of inches lost or gained when rounding individual measurements could, when summed, make a very real difference in comparison to what an overall measurement would yield. In construction, the placing and setting of things by measuring off a previously placed or set thing is common practice. To me, this is a relative way of placing, as opposed to an absolute way of placing, which would be to place or set things by measuring off the benchmark each time, which is turn would eliminate the accumulation of measurements and reduce the gathering of error.
However, it could be extremely tedious and not at all efficient to constantly measure off the benchmark. Nonetheless, to be exercising control, you must be at least regularly checking against the benchmark to ensure that you are not gathering measurement errors as the work progresses. All of that said, measuring instruments that enable an easy check to the project benchmark using laser technology or sonar technology is becoming more and more prevalent on job sites and is improving the accuracy and quality of the job sites that use them. *
Once a benchmark is set by locating a pin on the building site, that pin must then be correlated with the building site. In other words, the pin’s location in three dimensional space must be measured and associated with the known vertical measurements of the site, usually recorded in a surveyor’s map.
How are the vertical measurements of a surveyor’s map “known”?
In short, vertical measurements become known by measuring elevation off a mother-of-all-datums.
“Mother” used to be sea level, but these days she manifests more often as a geodetic reference system which, unlike plane surveying, takes the shape of the earth into account and therefore provides a much more precise accounting than sea level (it turns out the sea is not level at all).
Not to get confusing, but there are various universal datums that over time have been used to establish basis for vertical control. A survey map that is created and certified by a licensed surveyor is required to cite which datum the map is based on. So when you see contours or other elevations cited on a survey plan or topographic map, they are indicating heights that are measured off the datum that is cited by the survey, be it a geodetic system or if it’s an older survey then it may be sea level.
In any event, the base datum of the map will, logically, have a vertical value of zero. A contour or elevation of 200 feet is therefore representing a height of 200 feet above sea level or above the geodetic zero reference, whichever is cited by the map. If for some reason it becomes necessary (like when demonstrating the flood elevation), it is possible to mathematically convert one datum to another.
How Does the Actual Benchmark Pin Relate to the Site Map?
The “local” vertical measurements on a building site tie into that larger universal system of verticalmeasurement through the benchmark that becomes established on the site.
Let’s say the pin was pounded into a large tree trunk, at a height of two feet above the ground. If the ground in that exact location from which the two feet was measured represents an elevation of 150.375 feet on the surveyor’s map, then you could say that the benchmark of the site has been set at 150.375 + 2.0 = 152.375 feet.
Watch how cumbersome it can get: now if you place something by measuring 6.5 feet off the benchmark, it will have been placed at 158.875 feet. And then placing 3.5 feet off of that would be 162.375 feet. And so on, straight into a mathematical number-keeping nightmare.
For this reason, the project benchmark on the building site (the pin) often takes on an adopted elevation of zero feet / zero inches, to make the math more workable. You can always get back to the larger system by adding back the survey elevation of the site’s “zero-zero”.
Using the example above, a project elevation of 5.25 feet that is measured off the project’s adopted benchmark value of zero / zero would be at elevation 155.625 feet in the larger system (150.375 + 5.25 = 155.625). By converting the site benchmark to a zero value, do you see how less likely it would be for error to develop in the presence of all those larger numbers?
Setting the Pin with Intention
Better than being arbitrary in setting the pin that will identify the benchmark, it is wise to establish the benchmark with intention. A better way than arbitrary is to find where a meaningful datum occurs in the site and pound the pin exactly there.
For instance, the level of the first floor is very often set by the architect as the benchmark datum, or zero / zero, to the project’s system of vertical dimensions, with everything being then referenced to that datum. If the first floor is designed to be 2.5 feet off the adjacent grade, and grade is designed to relate with elevation 214 feet, then the benchmark pin would be pounded at elevation 214 + 2.5 = 216.5 feet. With the pin “programmed” to the design’s dimensioning system, everyone on the job site understands that the pin represents the level of the first floor, and the relationship of the design to its construction becomes that much better controlled by the building crew because they don’t have to do the math of factoring in the value of an arbitrary benchmark.
Units: Decimal Feet Vs Feet and Inches
Various unit types are employed by different design disciplines. This can irk the user of design drawings. Surveyors and engineers tend to record measurements, elevations and dimensions in decimal feet, as I have done in my examples above. Architects, on the other hand, tend to use feet and inches. So a measurement of three feet, six and one half inches would be represented by each group as follows:
Engineers and surveyors: 3.54166′
Architects: 3′-6 1/2″
No big deal, but it does require a little extra reflection when considering how the architect’s dimensions covert to the engineer’s, and vice versa. It behooves one to be not so quick in one’s calculations. For instance, if 6 inches = .5 feet, a quick think might suggest that six and one half inches would equal .55 feet. But that would be wrong. It is actually .541 feet. Not a big discrepancy, but again, small gathering errors can net into substantial trouble.
Here is how that figures: because there are 12 inches in a foot, an inch is 1/12 of a foot, which is .0833 feet. Half of that (one half of an inch) is .04166. So, six and one half inches is represented in decimal as .5 feet plus .04166 feet which equals .54166 feet.
A simple conversion tool always helps. If you google “convert inches to decimal feet”, numerous conversion charts and calculators will be available to you.
Related Topics
Generally speaking, there are datums in the realm of a design and construction project that are very charged with relevance in terms of land use regulations. A full discussion of these datums warrants a larger article, as we are coming to the end of this article. Some of those datums include:
- the Base Flood Elevation
- the Design Flood Elevation
- the Coastal Boundary
That larger discussion will be coming in the future … please consider signing up to receive email alerts to new articles if you have enjoyed this article and think you might be welcoming of future ones.
And if you have anything on your mind after reading what’s here and you wish to share those thoughts, please do not hesitate to leave a comment! Thanks!