Oblique imagery contains information not available from vertical imagery, such as building facades, points of ingress and egress, and profiles of features and objects. Oblique imagery is a more natural way of observing the world from the human point of view. As such, it is useful for manual image interpretation and for collecting and recording information about features. An important capability of oblique imagery is the ability to create and edit features in image space and save them in the map projection of choice.
The Image Space Analysis capability in the Image Analyst extension allows you to view and work with imagery collected at oblique angles by displaying it in an easy-to-understand manner. It is displayed with buildings and features oriented vertically toward the top of the display, which better enables image-interpretation applications. It also provides access to the imagery as it was collected, without distortion.
The map view display is transformed into perspective mode, where images are presented in image space (in columns and rows) rather than map space (in a map projection system) using an image coordinate system (ICS). The ICS facilitates the seamless transformation between image space and map space and allows additional image and GIS layers to be properly registered to the imagery. The ICS uses the rational polynomial coefficient (RPC) model, which describes the image orientation and position information, along with other pertinent information about how and when the image was collected to support the transformation between image space and map space. This model is typically imbedded in the image file or included as a separate metadata file. Enabling the ICS for imagery in the map view display is referred to as perspective mode.
Interpret images in perspective mode
The primary image that defines the ICS of the perspective mode is referred to as the focus image. There can be only one focus image in perspective mode of the map view at any given time, since the geometry of the focus image establishes the geometry of the perspective mode of the map view. Other images and GIS layers can also be displayed in the same view—they are simply transformed to the focus image geometry.
With the Image Analyst extension enabled, display any number of oblique images containing rational polynomial coefficients (RPCs) in the display map view.
The images are displayed in map view with north oriented toward the top of the display.
Satellite image courtesy of DigitalGlobe.
To establish a focus image, click List By Perspective Imagery in the Contents pane.
Once activated, your contents will be organized so that the images eligible for display in perspective mode are listed in the Contents pane in the Focusable Images group. These images are followed by other content in the Contents pane. This allows you to quickly identify oblique imagery and switch them to image space, which is especially useful for monitoring applications using multiple images.
Each focusable image in the Contents pane has a raster icon indicating whether it is in map space or image space. Hover over the icon next to the image name to display a pop-up message, Set as focus image. Click the raster button of the oblique image with which you want to work. The icon changes to indicate the chosen image is enabled and displayed in image space in the map view display.
The geometry of the map view display in perspective mode is now in the ICS geometry of the chosen image. Any other imagery and display content listed in the Contents pane will switch to the same ICS in image space. The oblique image is displayed in perspective mode. Satellite image courtesy of DigitalGlobe.
Note:
When the map view display is in ICS, also called perspective mode, coordinates of the pointer are shown as map space coordinates as well as image space coordinates in row and column location in the bar at the bottom of the map view.
As soon as the map view display switches to perspective mode, a heads up display (HUD) window appears that contains essential metadata for the perspective image that is helpful for reviewing multiple perspective images. The following screen capture shows metadata from an image in NITF format. The window is translucent until the pointer is placed in the window; it then becomes opaque to obscure the image in the background. When the pointer is near the edge of the window, it changes to a form that allows you to move the window in the map view. The window can be minimized by clicking the arrow in the upper right corner of the window.
Note:
When working with the List By Perspective Imagery tab in the Contents pane, images with sensor model information are collected into a special group layer named Focusable Images. The images listed in this group can then be displayed in perspective mode to support image interpretation workflows.
However, if images with sensor model information are in a group layer with other images or raster layers that do not have sensor model information—for example, a WV02 satellite scene grouped with a hillshade layer—the images with sensor model information will not be listed in the Focusable Images group layer when the List By Perspective Imagery tab is selected. In this case, it is recommended that any sensor-based images to be viewed in perspective mode are only grouped with other sensor-based images.
Increase the accuracy of the ICS and derived feature data and measurements by assigning a DEM to the transformation.
Right-click a focusable image and click Edit Function Chain, which opens the Function Editor and displays the function chain associated with your focusable image.
Double-click the Geometry function in the chain to open the function and display parameters.
Click the Method drop-down arrow and choose Use DEM, which presents options to select a DEM from the Contents pane or browse to a DEM file.
Click OK.
The DEM is added as an input to the Geometric function in the chain and applied to the focusable image. Edit the focusable image RFT to use a DEM to increase accuracy.
Note:
A Raster Function Template (RFT) is automatically assigned to images in the Focusable Images group. The RFT supports the rigorous transformation that converts between the ICS and the map projection system.
To measure features in perspective mode, click the Imagery tab and choose the type of measurement you want to perform from the Mensuration group. Using the Mensuration Tools drop-down arrow, choose to measure a point, distance, or area. Measure the feature in the display in perspective mode.
The results of your measurements are listed in the Mensuration Results pane. See Image mensuration for more information about mensuration capabilities in ArcGIS Pro.
To create features in perspective mode, load and highlight a feature class geodatabase and click the Edit tab. Create or modify features in perspective mode image space using the standard tools. Remember that features elevated above the ground will appear to be displaced from their ground footprint due to perspective geometry; the taller the feature, the more height displacement will occur. When finished, save your features, which retain the map projection geometry specified in your geodatabase.
Use the Image Space Analysis capability to display and work with imagery in perspective mode to enable more effective and intuitive image analysis. This is facilitated with the ICS, which provides the rigorous transformation between image space and map space. You can perform mensuration in image space and collect and view results in map space. Additionally, features can be created and edited in image space using standard ArcGIS Pro tools, and the feature data can be saved in map space. The organization of oblique imagery in the Contents pane and the presentation of image information in the metadata window aid efficient interpretation and analysis of imagery for monitoring applications.
