Georeferencing is a term that is used a lot in surveying circles, especially as digital surveying technologies such as LiDAR have grown in popularity. In this blog, we’re delving into what Georeferencing actually means, how it works, and how OxTS can help you do it more efficiently than you might otherwise have thought possible.
Georeferencing: a definition
Georeferencing is something that you do to a dataset, usually survey data. Georeferencing is the process of aligning that data to a coordinate grid. Usually that coordinate grid is one such as latitude, longitude and altitude – a global system that allows you to locate the data on the earth. However, you can georeference data to a local coordinate system, which allows you to position it accurately in relation to a specific point or set of points, but not necessarily on the earth.
Why is it important?
Lots of data – especially survey data – is only any good if you can put that data in a geographic context.
Consider the example of an aerial survey of a town using a LiDAR scanner. A LiDAR scanner produces a collection of data points that represent the entire town, and using software you can use the points to take very accurate measurements – for instance, of the heights of buildings, or the widths of roads.
But if you want to measure the distance between the centre of the town and a point not on your survey – say, the coast – you need more information. How far is your data from the coast? You can’t tell based solely on the survey. The only way to get the answer you need is to georeference your survey data. Once your survey is put in a geographic context, you can take measurements by combining it with other map data.
If your survey is sufficiently large, then georeferencing can also be important for taking accurate measurements at all. This is because over lager distances, the curve of the earth can affect your measurements, and your survey data is unlikely to account for that if not georeferenced.
How do you georeference data?
There are a variety of methods for georeferencing, and they all have the same two basic steps:
- Capture accurate navigation data during your survey
- Combine that navigation data with the dataset you want to georeference
The most basic method of georeferencing, which is used in surveying, is ground control points, or GCPs. These are physical targets placed on the ground in your survey area, which are separately surveyed and so have a known position. When you come to process your data, the GCPs allow you to georeference your entire map.
While GCPs are relatively simple to set up, they come with drawbacks. They are time-consuming to set up, especially if you’re surveying over large areas. You also need to make sure you have enough of them to produce accurate results.
An alternative that’s proving popular for surveyors and others who need to georeference data is to track the location of the sensor gathering the data in real time. One of the most common technologies for this is an inertial navigation system, also known as an INS or GNSS/INS. A GNSS/INS contains two key pieces of technology. An inertial measurement unit, and two global navigation satellite system (GNSS) receivers. They measure a variety of things including position, velocity, heading, pitch and roll. OxTS makes a range of inertial navigation systems.
Learn more: What is an IMU?
The GNSS/INS is mounted near to the device gathering the survey data – this could be a LiDAR, camera, radar or similar. Throughout the survey it records the survey device’s position and orientation. It’s vital that the positions of both your GNSS/INS and your survey device are precisely measured and recorded, so that you can accurately combine the data after the mission.
Once the survey is complete, the position data is combined with the survey data – or georeferenced. Some methods require you to do some post-processing on the localisation data, known as PPK, to make sure it’s as accurate as possible. Other methods can do the same processing in real time, known as RTK. Either way, once the localisation data is synchronised with the survey data, you should have an accurately georeferenced dataset.
How OxTS helps with georeferencing
OxTS has been helping surveyors and other organisations that need to georeference their data in a number of ways.
Firstly, our GNSS/INS devices are designed to gather highly accurate position, navigation and timing data that’s ideal for georeferencing. We even have GNSS/INS models that are small and light enough to be used on drones while still producing great results. They are all capable of RTK accuracy, giving you high-quality results in real time.
Discover our GNSS/INS products
Secondly, we provide software that helps georeferencing professionals achieve the very best accuracy in their work:
- Our NAVsuite software contains powerful post-processing capabilities that maximise the accuracy of your location data, even in areas where GNSS signal may have dropped.
- Our OxTS Georeferencer software is designed to simplify the process of combining LiDAR and localisation data in just a few clicks. It also contains tools that help you accurately calibrate the relative positions of your LiDAR scanner and your GNSS/INS during the survey. That calibration helps produce the clearest possible point clouds, and demonstrably reduces blurring and ‘double vision’.
- If you’re using LiDAR to survey, you may also want to investigate OxTS LIO. OxTS LIO is a software feature that can be enabled on any OxTS GNSS/INS that uses distance information from a 360° field-of-view LiDAR sensor to calculate the relative velocity of a survey vehicle. The data is then used to constrain position drift in urban canyons allowing the user to receive more accurate data for longer periods when GNSS conditions are tough.
We hope this blog has given you a greater understanding of what georeferencing is. If you’d like to learn more, or explore OxTS georeferencing solutions, contact us using the form below to leave us a message.
