Horizontal and vertical offsets are one of the most common issues when processing Drone2Map projects with different coordinate system datums. Typically, if an output product appears many feet or meters above where it should be in relation to the elevation surface it is being displayed in, the most likely reason is inaccurate GPS. However, when you are defining coordinate systems from different datums and using the incorrect transformation, the same issue can occur.
Every Drone2Map project uses the process of Aerial Triangulation (AT) to align neighboring images and determine their location in three-dimensional space. In the AT process in Drone2Map, it is assumed that you correctly defined the coordinate systems before running the adjustment step. Typically, drone imagery is captured in common horizontal and vertical coordinate systems such as WGS84 and EGM 96, which is the image coordinate system. During the adjustment step, a projection is done on the adjusted images from the AT process to generate products, which uses the project coordinate system. If a transformation is required and a supporting transformation is found, the output products are automatically transformed to the defined coordinate systems.
When transforming between datums, both horizontal and vertical coordinate systems must be considered. For example, if you start with imagery that was captured in WGS84 for both horizontal and vertical coordinate systems and want the output vertical coordinate system to be NAVD 88 instead, you must also transform the horizontal coordinate system. NAVD 88 is derived from the North American Datum, or NAD. NAD and WGS reference the earth in different ways and using the incorrect datum for a specific coordinate system can cause offsets in the data. It is important to understand what the underlying datum the coordinate system you want to use is derived from. To learn more about how vertical coordinate systems work, see Vertical coordinate systems.
Apply transformations through project options
As long as the adjustment step has not been run on a project, you can change the image and project coordinate systems in use, as well as define a transformation using the project options window.
Transformations are sorted by their accuracy and suitability for the location of the data. If the transformation is composed of multiple steps, each is listed. By default, an optimal transformation based on accuracy and extent of the data is automatically suggested under the project processing options drop-down menu, but you can specify a different transformation if necessary. For example, if you are transforming the data from WGS84 to NAVD 88, there are different geoids that can be selected based on your preference.
To modify the transformation used in the project, follow these steps:
- On the Home tab, in the Processing group, click the Options button.
- Click the Coordinate Systems tab.
- From the Select Transformations drop-down menu, select a new transformation.
If no transformation is needed, the No datum transformation is needed message appears and the drop-down menu is locked.
If no transformation can be found, a message appears and it is recommended that the ArcGIS Coordinate System Data be installed to expand potential transformation options. Trying to process with no transformation defined when one is needed results in a dialog box asking you to confirm processing and informing you that results will likely be incorrect.
If the ArcGIS Coordinate System Data is installed and still no transformation can be found, a message appears indicating this.
- Click OK.
Apply transformations to a map
The Transformation tab on the Map Properties dialog box lists the geographic and vertical coordinate systems of the layers and the current geographic and vertical coordinate systems of the map.
Note:
Changing the map transformation is done on the fly and does not alter any data or apply to output products. If you need to transform an output product, it must be defined in the processing options.
The parameters listed under the transformation method vary depending on the method used by the transformation. Certain details such as the Accuracy and Area of Use values are only available for transformations installed with ArcGIS Drone2Map or with the ArcGIS Coordinate System Data.
Change or view a transformation
To view the transformations applied in a map, or to choose a different transformation, complete the following steps:
- Right-click the map or scene in the Contents pane and click Properties.
- On the Map Properties dialog box, click the Transformation tab.
- In the Layer and map section, click the name of a transformation in the center column of the table to open the menu and choose a different transformation.
Repeat this step as necessary for other coordinate systems used by layers in the map.
If the geographic coordinate systems of the layers and the map are identical, no transformation is necessary.
- Optionally, click the Details link to open the Transformation Details dialog box.
The details of the transformation, including the transformation direction, the source and target coordinate systems, and the method are listed. Depending on the transformation, additional details may also be listed.
- Click OK to apply the transformations and close the Map Properties dialog box.
Apply a custom vertical transformation
If you have a vertical transformation file (.vrt) that you want to use in ArcGIS Drone2Map, you can copy it to the custom transformations folder. It will appear as an option when choosing from the available transformations.
To apply a custom vertical transformation file, complete the following steps:
- Copy the .vrt file to the CustomTransformations folder.
The folder is under the following path: C:\Users\<your username>\AppData\Roaming\Esri\Drone2Map\ArcToolbox\CustomTransformations.
You must have hidden items set to be visible to see the AppData folder.
- Create a project in ArcGIS Drone2Map with the vertical coordinate systems you want to transform between.
- Click the Options button on the Home tab.
- Click the Coordinate systems tab.
- From the transformations drop-down menu, select the custom vertical coordinate system.
- Click Apply.
Upon processing the project the new custom transformation will be used for any product outputs.