Survey Markers and Benchmarks

Survey Markers and Benchmarks are flat markers often made of metal that are found embedded into stone, concrete, or asphalt.  Benchmarks are a type of survey markers whose elevations have been surveyed as accurately as possible based on the technology at the time.  Surveyors and GIS professionals use survey markers for groundtruthing in order to verify the positional and vertical accuracy of geographic data.

An Introduction to Survey Markers

Critical to mapping and surveying is the need to used fixed points that establish a grid or location to measure and map objects within. While it’s true that mapping and surveying have evolved significantly during the last few decades, with new tools, imagery, and analyses making it easier to record and quantify space, the requirement to employ survey markers is one crucial aspect that hasn’t changed.

Where Does the Term Benchmark Come From?

The term derives from the symbol used to mark locations using a bench-like symbology created first by the British Ordinance Survey. In fact, it was an arrow below a horizontal line that gave it the benchmark look.

The benchmark was used at least by the late 18^th century as a way to map the topography of Great Britain by placing fixed points of known elevation across the country, which enabled surveyors to then create one of the first highly accurate maps of a country.

Hewitt (2011) provides an overview of the history of the Ordinance Survey and the use of benchmarks, including a network of them that can be found across the UK today with many of these being more than 200 years old, that were the backbone of the accurate maps produced by Britain starting in the late 18^th century.

The basic concepts developed for benchmarks in the 18^th century has not changed, except perhaps the underlying technology that provides location and elevation data have certainly changed. For some, benchmarks might be losing their need, as modern technology begins to replace their basic function, but they still serve important roles and functions in the creation of local maps and survey plans, particularly in more remote regions. 

How are Benchmarks Used in For Finding Elevation?

In both land surveying and geodetic measurements of the Earth’s shape and surface, survey markers and benchmarks play an important role in providing location and elevation, or orthometric height often based on mean sea level for national grids, at a particular fixed point, which can then be used to create an overall map based on other points that reference the marker. Elevation and location could be provided using different reference elevation and coordinates, sometimes using relative or absolute systems that have known references for location and elevation.

Typically, a surveyor would establish a local marker that works within a network of markers or can be fixed to a well established datum (Gopi et al. 2008). Within a survey, a survey marker is often established on a permanent location that could be referred to by surveyors as needed to establish or check a surveyed area or map the area.

In many modern surveys, a total station, measuring angles and distances, would be used, with the survey marker providing the fixed point to base absolute measurements that allows a map to be fixed to a global location.

Theodolites are also used to provide horizontal and vertical angle measurements based on established points that use a survey marker. Modern theodolites use automated, digital instrumentation to make calculations between points using laser to get accurate measurements (Kavanagh 2009). In addition to using benchmarks to establish a fixed location for maps, with two or more benchmarks data on direction and distance can be established for objects. 

There are also many historical survey markers and many or even most countries have a national benchmark system that allows local or regional mapping and surveying efforts to use these markers to make maps.

To some extent, many survey markers have become out of date, as they are not based on more modern GPS or satellite updated information, which provide more up-to-date data as the Earth’s shape evolves. For instance, in the UK, there are somewhere on the order of about 500,000 national benchmarks, but these are mainly out of date and many are disappearing as they are not updated.

Today, global positioning satellite (GPS) data are used to update and establish digital-based survey markers. Physical survey markers have fixed written information on them.

One issue is the Earth is always changing shape, meaning that over time location and elevation data may not be accurate. By establishing a GPS and satellite-based system, periodic updates can be made to compensate.

Benchmarks and Survey Markers in the United States

In the United States, the National Geodetic Survey is responsible for maintaining a grid of fixed survey markers including benchmarks across the United States as part of a national coordinate system. Additionally, a geoid model is used to compute the shape of the Earth that can then be used to estimate true location and height.

The current model Geoid96 is computed and maintained at the National Geodetic Survey. Additionally, GPS data are used to check the model and remove errors at given positions (Seeber 2008). Effectively a combination of GPS and the Geoid96 model help to ensure that true location for fixed benchmarks maintains accuracy.

Currently, the National Spatial Reference System (NSRS) is modernizing and will create a national, GPS-based system that provides modern benchmarks with automated and more frequently updated coordinates as needed to users of the system. This project is projected to finish by 2022 (Geodesy 2021).

Spanning the last two hundred years and continuing to today in the United States, there are approximately 240,000 benchmarks, which include fixed GPS locations as well as older locations fixed previously.

The data can be accessed from the National Geodetic Survey Data Explorer maintained by the National Oceanic and Atmospheric Administration (NOAA 2021). Other agencies also maintain a network of benchmarks. This includes the United States Army Corps of Engineers, the United States Forest Service, and the United States Geological Survey.

What is a Witness Post?

Since survey markers are frequently small metal disc affixed to the ground in cement or onto rock, witness posts are signs that serve as a visual aid. Witness posts (or marks) can be signs on wooden or metal poles that let the viewer know that a survey marker is located nearby. Survey markers are usually placed within a few feet of the witness post.

Future of Benchmarks

Some would argue that the days of benchmarks could be ending, at least the old, non-GPS fixed points, as GPS stations and improved satellite-based systems make the need for land-based, fixed points less needed.

In the UK, the Ordinance Survey has not added a benchmark for over 25 years. However, there are still areas where GPS data may not be as accurate as one may want and fixed benchmarks do provide a way to create accurate local maps.

Historical Reasons for Maintaining Benchmarks

Another utility of benchmarks is for historical reasons. For instance, many old maps and surveys used benchmarks that still exist today. Features built or surveyed and mapped decades or even hundred of years ago could have disappeared. In order to reconstruct the mapped or surveyed area, one would need the benchmark present to reconstruct what the grid may have been for an area.

This could be true, for example, in property that has a long history and may have been surveyed decades or centuries ago and the only way to reconstruct the full extent of the property may require resurveying using old benchmarks. In fact, countries still have national laws that require the protection and access to land to enable surveyors to map and measure property, indicating the importance that benchmarks have on property (Int’L Business Publications 2009).

Even for critical infrastructure mapped decades ago, such as gas lines, these may have to be checked using old surveying marker data to certify their safety. Overall, older survey markers are disappearing in countries such as the UK and US. Nevertheless, they provide practical utility for historical infrastructure, while they are also part of our heritage that is worth preserving.

Despite the fact that some national-based benchmarks and other survey markers are disappearing, local and some regional benchmarks continue to be important. This means that for modern surveyors training in the field, they will still need to learn methods that require them to incorporate benchmarks in establishing grids and survey measurements in maps. While much of this is likely to be mostly automated, relative to early maps that were made by purely optical instruments, the skills required means that benchmarks are important to learn for any surveyor and play an important role in creating maps at local and even national scales.

The likelihood is that fixed point benchmarks will slowly fade as their need diminishes, but their historic interest could lead to some being preserved for many years to come. 

References

Geodesy. 2021. National Geodetic Survey. https://geodesy.noaa.gov/GPSonBM/

Gopi, Satheesh, R Sathikumar, and N Madhu. 2008. Advanced Surveying: Total Station, GIS and Remote Sensing. New Delhi: Dorling Kindersley.

Hewitt, Rachel. 2011. Map of a Nation: A Biography of the Ordnance Survey. Paperback ed. London: Granta.

Int’L Business Publications. 2009. Finland Land Ownership and Agriculture Laws Handbook. Place of publication not identified: Intl Business Pubns Usa.

Kavanagh, Barry F. 2009. Surveying: Principles and Applications. 8th ed. Upper Saddle River, N.J: Pearson/Prentice Hall.

National Oceanic and Atmospheric Administration. 2021. National Geodetic Survey Data Explorer.             https://geodesy.noaa.gov/datasheets/ngs_map/.

Seeber, Günter. 2008. Satellite Geodesy: Foundations, Methods, Applications. 2nd completely rev. and Extended ed. Berlin ; New York: Walter de Gruyter.