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, then the most likely culprit is inaccurate GPS. However, when you are defining coordinate systems from different datums and using the incorrect transformation, then the same issues can occur.
Every Drone2Map project uses the process of Aerial Triangulation (AT) to align neighboring images and determine their location in 3-dimensional space. The AT process in Drone2Map assumes that you have correctly defined your coordinate systems before running the adjustment step. Typically, drone imagery is captured in very common horizontal and vertical coordinate systems such as WGS 84 and EGM 96, this is the image coordinate system. During the adjustment step there is a projection that is done on the adjusted images from the AT process to generate products, this uses the project coordinate system. If a transformation is required and a supporting transformation is found, then the output products will automatically be transformed to the defined coordinate systems.
When transforming between datums both horizontal and vertical coordinate systems need to be considered. For example, if you start with imagery that was captured in WGS 84 for both horizontal and vertical coordinate systems and want the output vertical coordinate system to be NAVD 88 instead, then you are going to need to 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 your 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 yet on your project, you can freely change the image and project coordinate systems in use as well as define a transformation through the project options window.
Transformations are sorted by their accuracy and suitability for the location of your 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, but you can specify a different transformation if necessary. For example, if you are transforming the data from WGS 84 to NAVD 88 there are different geoids that can be selected based off your preference.
To modify the transformation used in the project, follow these steps:
- On the Home tab under the Processing group, click the Options button.
- Click the Coordinate Systems tab.
- Use the Select Transformations drop-down to select a new transformation.
If no transformation is needed a message of "No datum transformation is needed" will be displayed and the drop-down will be locked.
If no transformation can be found a message is displayed and it is recommended the ArcGIS Coordinate System Data be installed to expand potential transformation options. Trying to process with no transformation defined when one is needed will result 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, then a message will be displayed 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 needs to 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 Accuracy and Area of Use are only available for transformations installed with ArcGIS Drone2Map or with ArcGIS Coordinate System Data.
To view the transformations applied in a map, or to choose different transformations, follow these 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 method are listed. Depending on the transformation, additional details may be listed as well.
- Click OK to apply the transformations and close the Map Properties dialog box.