There’s a conspiracy theory that claims the Earth is flat, and interestingly, a 2019 YouGov poll found that 3% of Brits believe it to be true. If only that was the case, this would be a dream come true for Land Surveyors! A flat Earth would certainly make our jobs a whole lot simpler.
Sadly, reality doesn’t quite align with this belief. We know the Earth is round, and we have the tools to prove it. Using GNSS receivers and total stations, we must account for the Earth’s curvature in our surveys to ensure both accuracy and consistency. Without this crucial adjustment, discrepancies would quickly spiral out of control, especially over large distances. To correct these distortions, we apply a scale factor to maintain accurate and precise measurements.
What is the Scale Factor?
In simple terms, the scale factor is a numerical adjustment that helps translate real-world distances on the Earth’s curved surface to a flat map or survey grid. To be specific, because the Earth is an oblate spheroid, the distances and angles we measure on the ground don’t directly match those on a two-dimensional representation. The scale factor compensates for this curvature, ensuring that survey data remains accurate and reliable.
How does it work in the UK?
In the UK, the scale factor plays a crucial role in national mapping systems, particularly in the Ordnance Survey National Grid. Because the Earth’s surface curves, a Transverse Mercator projection is used, with the central meridian having a scale factor of 0.9996012717. This minimizes distortion across the country. As you move further East or West from this central meridian, the scale factor increases slightly. By applying these adjustments, we ensure that measured distances on the ground align accurately with those on official maps and the OSGB36 coordinate system.
How big is the error if not applied?
Failing to account for the scale factor can introduce significant errors in survey measurements. For example, from XRS Geomatics' office in Salisbury, an uncorrected scale factor results in an error of approximately 40 cm for every 1 km measured. Over longer distances, this discrepancy compounds, meaning a 10 km survey could be off by around 4 meters. This level of inaccuracy is unacceptable in professional surveying, which is why applying the correct scale factor is essential for maintaining precision in mapping and construction projects.
How do I find the scale factor for my area?
Several software applications can calculate the scale factor for your location. Ordnance Survey offers a tool to assist with this, which you can find more details about here: https://www.ordnancesurvey.co.uk/products/os-net/coordinates
Additionally, NG Surveys Ltd developed an Apple and Android app that determines the scale factor by entering your postcode or known coordinates. It can be downloaded here: https://www.nrgsurveys.co.uk/LocalScaleFactor.html
Happy Surveying!
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