Create a ground control points set

In Site Scan, ground control points (GCPs) are saved at the project level as a GCP set. One set can be used to georeference multiple missions within a project.

A GCP set is a collection of ground target points surveyed on the site location. You can upload the GCPs to Site Scan in a .csv or a text (.txt) file. The file must be formatted in columns. Each point must have the following information: Point Number, X-coordinate, Y-coordinate, Z-coordinate, Description (optional). Any additional columns will be discarded.

GCP set coordinate systems

The GCPs collected are always associated with a coordinate system. A coordinate system is a reference system used to represent the locations of geographic features. The GPS locations of your GCPs should all be surveyed within a common geographic framework.

To simplify the use of coordinate systems, Site Scan has two categories: known coordinate systems and custom coordinate systems.

Known coordinate systems

These are coordinate systems whose framework is well documented, and their information is widely available. There are thousands of known coordinate systems in the world. Site Scan has a database of known coordinate systems available for your use.

Custom coordinate systems

Custom coordinate systems are those that do not exist in the Site Scan database. These can be local grid systems or variants of known coordinate systems. If your GCPs are not in a known coordinate system, you can set up a custom coordinate system in Site Scan so you can still process and analyze your data.

Add a GCP set with a known coordinate system

To add a GCP set with a known coordinate system in Site Scan Manager, complete the following steps:

1. Open the project which you want to add the GCP set. On the left, click the GCPs tab.
2. Click Upload GCP Set.
3. In the new window, in the GCP Set Name field, type a unique name for the set.

   Note:

   The current date is automatically associated to the GCP set.

4. Click Import GCPs, browse for the .csv or .txt file on your computer, and click Open.
5. Confirm that the columns assigned in the preview window are in the right order with respect to your GCP file.
   1. If the columns are reversed, you may switch them by clicking the column headers and changing them.
6. If the GCP coordinate system is the same as your project, choose Same as current project settings and skip to step 10. If the GCPs are in a different coordinate system, select Known Coordinate System.
7. In the Known Coordinate System field, search for your coordinate system using either of these methods:
   1. Type the name of the coordinate system and click it from the list.
   2. Type the EPSG code and click the corresponding coordinate system from the list.

      Note:

      If your coordinate system does not appear, use the custom coordinate system method below.

8. For Vertical Units (if applicable for the selected coordinate system), from the drop-down list, select Meters, Standard Foot, or U.S. Survey Foot. This should be the units in which the GCPs were collected.
9. For Vertical Datum, choose the vertical datum in which your ground control points were collected.
10. Verify you don't have any errors, and click Save.
11. If you chose a different coordinate system for the GCP set than the project coordinate system, you will be prompted to change the project coordinate system to match the GCP set coordinate system upon saving the GCP set. While it is not required that the coordinate systems match, maintaining coordinate system consistency simplifies data processing by avoiding transformations. Click Yes (recommended) to change the project coordinate system to match the GCP set coordinate system.

Note:

To help ensure that you've properly selected the Northing (Y) and Easting (X) columns, Site Scan will check that your GCPs fall within 60 miles (100 kilometers) of your defined project location. If you see an error message, try swapping your X and Y columns, or verify that your project location has been set correctly. From the Project List view, click the Options button for the desired project, click Edit > Next to edit the project location.

Add a GCP set with a custom coordinate system

This method requires locating the GCP on a map. For best results, it helps to use a previously processed drone image orthomosaic even if it was not processed with GCPs because it provides better detail than a standard satellite imagery basemap. Better image detail allows you to more accurately locate the GCP on the map. However, if you do not have a drone image orthomosaic, using a satellite basemap is acceptable. You will specify one or two of your GCPs on the image map. Using this information, Site Scan will create a custom coordinate system over the 2D map using the rest of the GCPs in the set.

1. Open the project to which you want to add the GCP set. On the left, click the GCPs tab.
2. Click Upload GCP Set.
3. In the new window, in the GCP Set Name field, type a distinctive name for the set.

   Note:

   The current date is automatically associated with the GCP set.

4. Click Import GCPs, browse for the .csv or .txt file in your computer and click Open.
5. Confirm that the columns assigned in the preview window are in the right order with respect to your GCPs.
   1. If the columns are reversed, you may switch them by clicking the column headers and change them.
6. If you have already defined a custom coordinate system for the project and want to use that for the GCP set, choose Same as current project settings and skip to step 11. If you want to define a new custom coordinate system for the GCP set, choose Custom Coordinate System.
7. For Vertical Datum, choose the vertical datum in which your ground control points were collected.
8. For Horizontal Coordinate Unit, choose the units in which your GCPs were collected.
9. For Geolocate GCP, do the following:
   1. Select a GCP to locate on a map, and click Geolocate GCP.
   2. From the menu on the top, select the mission you want to use to geolocate.

      It is recommended that you use a processed orthomosaic from drone imagery for a clearer image.

   3. Zoom in and find the ground target that belongs to the GCP you selected, and click the center of it to place a tag.
   4. Click Save.

      If Rotated from True North is set to No (Default), you only need to geolocate one GCP.

      If Rotated from True North is set to Yes, you need to geolocate two GCPs.

10. Verify that you don't have any errors and click Save.

    Note:

    If you chose a different coordinate system for the GCP set than the project coordinate system, you will be prompted to change the project coordinate system to match the GCP set coordinate system upon saving the GCP set. While it is not required that the coordinate systems match, maintaining coordinate system consistency simplifies data processing by avoiding transformations. Click Yes (recommended) to change the project coordinate system to match the GCP set coordinate system.

Add more points to a project with an existing GCP set

In some cases, you may need to add more points when older ground marks have been covered, destroyed, or removed.

When adding new points using a known coordinate system, you may add points to an existing GCP set. If there are individual GCPs that are no longer usable, you can create a new GCP set following the previous steps that excludes the unusable GCPs. You cannot delete individual GCPs from a GCP set.

For custom coordinate systems, you can either add points to the original GCP set, or create a new GCP set. When creating a GCP set, select the Same as current project settings option under the Coordinate System section. This will allow you to use the previously created custom coordinate system and you won't have to geolocate GCPs again.