Remote Sensing

Remotely sensed data have played an important role in documenting the condition of our landscape and in monitoring changes and trends in those conditions. Historically, aerial photography was the primary source for characterizing the contiguous landscape and events occurring on that landscape. While photography remains a critical resource, other data captured from a variety of sources are also used to map and monitor vegetation and land cover. These data are captured from a variety of platforms ranging from fixed ground stations to satellite based sensors, and are entirely digital in their format. In assessing land cover with remotely sensed data, it is now common to make use of several types and scales of imagery to achieve the desired product.

The Pacific Southwest Region of the Forest Service relies on several data types and sources in its efforts to map and monitor forestland resources. Hierarchically, these data range in scale from coarse to fine, with widely varying spatial and spectral resolutions. The scale and resolution of the data that are used depends on the scale of the information need balanced by the time and cost necessary to process those data. For example, a landscape scale vegetation map that covers 50,000 hectares is typically derived from mid-resolution satellite imagery while a finer scale vegetation map covering a single small watershed is more appropriately derived from large scale, high resolution satellite imagery or digital photography.

Scale and resolution are two concepts critical to the evaluation and use of digital remotely sensed data. While linked, the two terms are not synonymous. It is possible, if generally inappropriate, to view low resolution imagery at a large scale and, conversely, to view high resolution images at small scales. The result can be inadequate information in the source data or far more information than is necessary, resulting in greater processing costs and time. Balancing the scale and resolution of source data with the information need, while a simple concept, is frequently a challenge to accomplish. In the course of meeting that challenge, the Pacific Southwest Region has acquired and archived a variety of digital remotely sensed data sources. These data represent a broad range of remotely sensed data available, but are only a subset of the many image types and sensors currently available on the commercial market. Detailed descriptions of most digital imagery are easily obtained on the World Wide Web. The following illustrates examples of data that have been and are commonly used by the Pacific Southwest Region to obtain information about various natural resources.