Overview of Methods
A view from orbit
Remote sensing is the science of obtaining information about an object or area through the analysis of measurements made at a distance from the object (i.e., not coming in contact with it). The oldest form of remote sensing is aerial photography where the sensor system is the camera and film. More recently, the field of remote sensing has grown to include electronic-optical sensors which acquire multispectral digital images that can be processed and analyzed by computers. Many of these sensors are on satellites which regularly orbit the earth.
19 Landsat images cover Minnesota
The Landsat satellite orbits the Earth 438 miles above the surface. Its sensor, called a multispectral scanner, records images of the same 115-mile wide path every 16 days. Nineteen images from five orbits are needed to cover Minnesota . The smallest area recorded is a ground resolution cell or pixel in the imagery measuring 30 x 30 meters (about 1/4 acre). The scanner records digital images of the surface reflectance in visible and infrared wavelengths of the electromagnetic spectrum. The infrared spectral bands are especially useful for mapping vegetation.
The quantity most frequently measured and recorded in images is the electromagnetic energy reflected by the object. The source of the electromagnetic energy is the sun and the spectral reflectance properties of many Earth surface features, such as soil, vegetation and water, can be used to uniquely identify and characterize them.
For all our satellite-derived mapping applications, an analyst needs to determine the relationship between the intensity of reflected wavelengths and a set of "calibration" sites. For impervious surface mapping, these sites represent the true, on the ground percentage of impervious surface area. For land cover mapping, these sites represent known examples of different classes of landscape cover (e.g., forest, agriculture, water, etc.). The relationship that is determined between the calibration sites and the electromagnetic response is then applied to all the pixels in the image, providing an image-wide classification or map.
The following links provide a more in-depth look into our methods for producing the maps found on this website:
The sensors carried on satellites can view portions of the electromagnetic spectrum that are both visible and invisible to the human eye.
Maps can be made by determining recognizeable signatures from Landsat imagery for land cover types and levels of imperviousness.