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Understanding localization: how to define a local coordinate system by known points

Localization in Emlid Flow

Have you ever surveyed a site without accurate coordinate system parameters or with none at all? The new localization feature in Emlid Flow can help you calculate or adjust local coordinate system parameters and enable you to survey the area with high accuracy. In this article, we’ll explain what exactly localization is, how you can benefit from it, and how it works in Emlid Flow.


What is localization?

One of the starting points of any survey is to choose a suitable local coordinate system for the area you want to survey. In most cases, coordinate systems are optimized for that particular location. However, sometimes this just doesn’t work. You simply don’t have the necessary coordinate system parameters, or they are not accurate enough. This is where localization comes in: the only requirement for this is the availability and applicability of benchmarks or known points.

Localization, also known as site calibration, is the process of comparing sets of known and measured point coordinates in GNSS software to calculate or adjust the local coordinate system parameters. Known points must have local coordinates (measured in feet or meters), and measured points (as global coordinates in degrees).

Pair of points for localization

Known and measured points for localization

During the calculation process, the software applies a series of transformations such as translation, rotation, scaling for horizontal adjustment, and inclined plane for vertical adjustment. The app then creates the best possible site reference frame, i.e. a coordinate system.


What data do you need for localization?

The localization process and its accuracy heavily depend on the technical assignment and data you have. You should carefully consider the availability of coordinate system parameters, the type of adjustment you want and can make, and most importantly the number, distribution, and quality of control points.

Coordinate system parameters

You can create a project with or without coordinate system parameters: a projection, a vertical datum, and a datum transformation. If you have the parameters but they aren’t accurate, it’s still better to use them than to ignore them. In general, you can try all possible variations and compare the results.

Adjustment types

The adjustment can be in plan or in elevation—or both. It all depends on the known point coordinates, their availability, and their quality.

Optimal number and distribution of known points

The number of known points has a direct effect on your survey accuracy. In general, 5 to 10 points evenly distributed over the survey area are optimal. The rule of thumb is to perform localization for smaller sites and collect measurements only within the area where the known points are located.

Points distribution for localization

Points distribution for localization


How does localization work in the Emlid Flow app?

Localization in Emlid Flow has a simple workflow with the all-in-one project option, so you don’t need to switch between projects. It also features a fine-tuned calculation algorithm. Once you enter all your data, the app will automatically pick the ideal algorithm to calculate the coordinate system parameters. 

After adding pairs of known and measured points and a series of mathematical calculations, you get the residuals. These are horizontal and vertical “errors” for each pair of points that allow you to assess the accuracy of the resulting localization. In Emlid Flow, you can easily turn horizontal and vertical point coordinates on or off; you can also remove coordinates that clearly stand out from the rest to achieve the desired accuracy. All these things can be done on a single screen, so you can watch the estimated accuracy changes in real-time.

Residuals of localization

Enable/disable coordinates for accurate localization

Get started with localization in the Emlid Flow app in just a few steps:

  1. Open an existing project or create a new one. If you want to create a new project, specify the known coordinate system that needs to be adjusted or use the Global CS option.
  2. Tap Project localization.
  3. Add pairs of control and measured points.
  4. Analyze the residuals.
  5. Preview results and apply them to the project. That’s it!

Learn more in our step-by-step guide.


Try localization with Emlid Flow

Localization is the perfect choice when you want to work in the local coordinate system but don’t have the parameters you need (or when you have to adjust them to fit the survey site). All you need are the control points at hand, Emlid Flow and Reach. Simply start your usual survey workflow and let Emlid Flow calculate the parameters of the required local system for you. Our new localization feature is available with the Survey Plan subscription in Emlid Flow. Try it now with a free 30-day trial.



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