To create a map of areas of a field that match a designated spectral signature, follow these seven steps:
- Set image orientation to As Collected
- Identify an image with your feature of interest
- Activate the Image Window and press F7 to adjust the filter settings
- Identify an area of interest in the Image Window by holding CTRL, then left clicking and dragging your mouse to create a rectangle around the content
- Create an Area Density or Stress Map, or filter out content by Absolute or Relative spectral relation to your highlighted region
- Adjust the strength of the new Custom Filter
- Recalculate maps to include the new Custom Filter
The following Custom Filter demonstration is an example of creating an Area Density map of dodder weed in an alfalfa field:
Set image orientation to As Collected
Begin by setting the image orientation to “as collected,” by clicking Display—Image View—Orientation—As collected, as shown in Figure 1. The default Image Window view is set as North-up to align the images with the Map Window view which is always oriented North-up. When the Image view is changed from North-up to As collected, images will be rotated such that the top of the image in the Image Window faces the front of the aircraft at the time the image was captured. For example, if the flight legs were flown East-West, the images taking while flying East to West will be rotated 90 degrees counter-clockwise relative to the North-up view, and the images flown West to East will be rotated 90 degrees clockwise relative to North up.
Image orientation will not change until a new image footprint is highlighted in the Map Window. If the orientation was changed while active in the Image Window (look for the red border), double-click or press the Tab key to activate the Map Window and move your cursor to a new image footprint. The Image Window will update the orientation, and you can proceed with creating the Custom Filter.
Identify an image with your feature of interest
Double-click on an image footprint in the Map Window. In the Image Window, zoom in on the feature and press the F7 key. Adjust Shadow and Soil Filters if the content you intend to select is already partially filtered out. A Custom Filter cannot be created if the Custom Filter Region is already filtered out by Shadow and Soil Filters. It is easiest to create a Custom Filter and vary its strength while the Shadowand Soil Filters are set to zero. You can always return the Shadow and Soil Filters to their previous values once the Custom Filter is set.
Create the Custom Filter Region
While holding down the control key, left-click and hold the mouse button to draw a rectangular box around the feature of interest. Figure 2 shows a box drawn over a patch of dodder weed in alfalfa.
Create an area density map
SlantView isolates points throughout the Dataset with reflectance spectra matching that of the highlighted box. To create a map of all these areas, right click inside the box and select Create map—Area Density, as shown in Figure 3.
Similar to the Soil and Shadow Filters, the image content identified by the Custom Filter Region is covered with solid grey pixels.
If Soil, Shadow, or other Custom Filters are active, there may be several grey-ish colors overlaid on your image. If you cannot see the Soil, Shadow, or Custom Filtered pixels, press the F7 key to activate the Filter View in the Image Window.
Adjust the strength of the Custom Filter
To adjust the strength of the Custom Filter, bring your cursor inside the box and hold the control key while scrolling up or down with your mouse wheel. Shadow and Soil Filters can be adjusted alongside the Custom Filter by bringing the cursor outside the Custom Filter Region and holding Ctrl or Ctrl+Shiftkeys and scrolling the mouse wheel, as described in section Adjusting Shadow and Soil Filtering Settings. The image on the left of Figure 4 shows a weaker Custom Filter; the image on the right shows a stronger Custom Filter that has identified more pixels, and effectively highlighted all the dodder weed in the image. Zoom out and move around the images in the Image Window when increasing and decreasing the strength of the Custom Filter to see how other areas are affected.
After setting an appropriate Custom Filter strength, return the Shadow and Soil Filters to their previous levels, and click the Recalculate maps button on the far right of the toolbar, or in the settings menu to generate the Area density map. The new map layer will be named Custom-Density by default. The Custom Filter map will show up as a new tab at the bottom of the list in the top right of the Map Window, seen in Figure 5.
To edit a Custom filter, you must navigate back to the red box of the Custom Filter Region where it was created. To save time, consider leaving an annotation on the image, explained in the section View Data. Alternatively, if you need to delete an old Custom Filter that you are having trouble finding, simply draw a new box anywhere, right click and select Delete all filters, then Recalculate the maps.
Four different filtering options including the Area Density map can be used in SlantView:
- Filter out—Absolute
An absolute Custom Filter removes the highlighted pixels from the map products in the same way as Shadow and Soil Filters.
- Filter out—Relative
The Relative filter functions the same way as the Absolute filter, but uses normalized rather than absolute reflectance data. In general, this mode is useful in fields with different levels of illumination, such as tall crops (e.g. vineyards and orchards) that cast shadows on the data you want to filter out.
- Create Custom Stress Map
After drawing a Custom Filter Region, right click—Create map—Stress. If using the Custom Filterto highlight plant content rather than content to be removed (soil, weed, etc.), this option will make a Stress map that includes only the content that was highlighted by the Custom Filter.
- Create Custom Area Density Map
As seen in the tutorial above, the Area density Custom Filter creates a map matching what is highlighted in the Custom Filter Region.
The Soil and Shadow Filters, and the Filter out and Create map Custom Filters are NOT mutually exclusive. The Filter out—Relative or Absolute filters can include the same content as custom Area density and Stress maps, and Soil and Shadow Filters.