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Set up custom coordinate system

tip

We recommend setting up the projects indoors before the survey as some coordinate systems require additional downloads in Emlid Flow.

This guide describes how to set up a custom coordinate system in Emlid Flow or Emlid Flow 360 if you cannot find the required one and know its parameters.

Overview​

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You can only configure the coordinate system while creating a new project.

To configure the custom coordinate system, you can use 4 different properties:

  • Ellipsoid your datum is based on
  • Projection type and its parameters
  • Transformation type and its parameters
  • Geoid model
note

Since not every group of parameters is required for a coordinate system configuration, you may omit some of them and only use the parameters that you have for your projection.

  • If the projection is based on the same datum as the base, skip the transformation step.
  • If you decide to work with ellipsoidal height, don’t select the geoid model.
tip

On some Android devices, default keyboards do not provide a minus sign. To get access to it, we recommend downloading Gboard, a virtual keyboard app, from the Google Play Store.

To show you all the steps, we will walk you through the example of transformation from the GDA2020 datum to GDA94, performing MGA zone 51 projection, and applying the AUSgeoid2020 geoid model to the GDA2020 datum.

Workflow​

Creating custom CS​

When picking a coordinate system for your project, follow the steps below:

  1. Tap the Coordinate system section.

  2. To create a custom coordinate system, tap the Plus button in the top right corner.

  3. Enter the name of your custom coordinate system. Using this name, you will be able to find it in the list of coordinate systems later on.

    We name the coordinate system GDA94 / MGA zone 51 [GDA2020 base].

Specifying ellipsoid for datum​

note

This step is mandatory.

In this section, we set up the ellipsoid the datum based on. By default, the GRS 1980 ellipsoid is selected. To change it, perform the following steps:

  1. Go to the Datum screen.

  2. Tap the Ellipsoid cell and choose the required ellipsoid from the list. It will update the Semi-major axis and 1/f parameters accordingly.

    The GDA94 datum uses the GRS 1980 ellipsoid, so we leave it as is.

  3. Tap Save to apply changes.

note

If the required datum is missing, contact us at support@emlid.com.

Applying projection​

  1. Go to the Projection screen.

  2. Choose the required projection type:

    • Albers Equal Area Conic
    • Krovak
    • Lambert Conic Conformal (1SP)
    • Lambert Conic Conformal (2SP)
    • Mercator (Variant B)
    • Oblique Mercator (Variant A)
    • Oblique Mercator (Variant B)
    • Oblique Stereographic Alternative
    • Transverse Mercator

    We perform the Transverse Mercator projection.

  3. Enter the projection parameters.

  4. Save changes.

Specifying transformation parameters​

note

You need to configure this option only if your coordinate system uses a datum that differs from the base's one.

  1. Go to the Transformation screen and turn it on.

  2. Choose the required transformation type: Helmert or Molodensky-Badekas.

    We need the Helmert transformation to go from GDA2020 to GDA94 datum.

  3. Fill in the transformation parameters: there are 7 of them for the Helmert transformation and 10 for Molodensky-Badekas.

    tip

    The app can also perform transformations with only 3, 4, and 6 parameters. You can type in the parameters you have and leave the others empty.

  4. Save changes.

Choosing geoid model​

note

Apply the geoid model to your custom coordinate system only if you require orthometric height for your project. Emlid Flow applies geoid correction to the datum the base works with, so make sure to choose the appropriate one.

  1. Go to the Geoid screen and turn it on.

  2. Click on the Model and select the appropriate geoid file. The app will suggest you download the geoid model if you have not done it earlier.

    We select the AUSgeoid2020 that is meant to be used for coordinates in GDA2020.

  3. Once the geoid is downloaded, tap Save.

Saving custom CS​

Once everything is configured, tap Save on the New CS screen to save the coordinate system. From now on, your custom coordinate system is saved to your Emlid account and synced among all your devices.

tip

You can find your custom coordinate systems later while configuring new projects.

You can continue creating the project for survey.

Editing custom CS​

You can edit parameters of your custom CS in the Emlid Flow 360 app. To do so, follow these steps:

note

The editing option is not available for calibrated systems.

  1. Tap Profile in the upper-left corner and select My coordinate systems from the drop-down menu. The list of your custom CSs will open.

  2. Tap the three dots button to rename, duplicate, edit or delete your custom CS.

tip

Duplicating your CS allows you to quickly edit parameters without having to enter them twice.

  1. Edit the parameters and tap Create coordinate system.

Further reading​