Choosing the right image to order
This article covers:
Spatial Resolution Considerations
The variety of options means greater coverage for each of your properties, but it can sometimes be difficult to determine what spatial resolution is sufficient for various monitoring purposes. Spatial resolution refers to the size of the smallest feature that’s visible in an image. So when we refer to a 0.5-meter resolution image in Lens, that means that one pixel in the image corresponds to a half meter by half meter area on the ground. The chart below allows you to compare various spatial resolutions for different property sizes to give you a sense of how detailed the image will be.
The Nature Conservancy released a white paper in the fall of 2020 entitled " Remote Property Monitoring at The Nature Conservancy in California" about their experience navigating different remote monitoring options. They developed a table with guidance on what level of spatial resolution is needed to detect various types of changes on conservation easements. The table provides examples of permitted or prohibited uses and activities, and their findings for the minimum spatial resolution required to detect various changes for reference.
Timing is an important factor in choosing an image that meets your needs. As landscapes change seasonally and sunlight shifts throughout the day, captured images subsequently change in appearance. Sometimes lack of imagery availability will narrow your options, but when presented with multiple image choices, here are some things to consider.
When monitoring areas with dense deciduous forests, ordering imagery before leaves appear in the spring or after they drop in the fall will allow for clearer sight of structures and activities beneath forest canopy. Using the public Sentinel-2 vegetation data, you can determine when leaf-out and leaf-off occurs on your property. Alternatively, areas of tree removal and forestry work can be most easily spotted in peak-leaf imagery, as any new voids in vegetation will stand out when compared to past seasons.
When previewing an image to order, note the time of day that the image was captured. Images later in the day, especially those captured in the northern hemisphere late in the year, may have more shadows from trees and structures. For some types of monitoring, those shadows may be distracting. Or, shadows may work to your advantage--for instance trying to spot a shed in a snow covered field that may otherwise blend in.
For more guidance on how often and when imagery becomes available, check out our article here.