Lab 8: Radiometric Signatures

Lab eight was designed to introduce students to the concept of how objects have unique radiometric signatures. Different mediums of material all have a unique reflective value across all the bands meaning that they can be classified via their radiometric values. While I do not go as far as to automatically classify features in this lab I do go through the process of collecting the radiometric signatures of different types of land cover.

All the data for this lab was provided by the Earth Resources Observation and Science Center, United States Geological Survey

In order to start this process I needed to find features and record their reflectance. I used the spectroradiometer tool included in the Erdas interface to record the signature of about a dozen different feature types such as standing/moving water, vegetation, crops, and urban features. This operation can be done at a smaller, more accurate scale in the field with a physical spectroradiometer but recording the signatures with this program will work just fine for my application.

Figure 1. Radiometric signature of standing water (left window). Notice the comparatively high reflectance of bands 1-3 when compared to bands 4-6. 

Standing water was the first feature which I recorded a radiometric signature. If we look at the signature mean plot, as seen in the above image, we notice that the highest levels of reflectance can been seen in the first three bands of the spectrum; blue being the highest with green and red having slightly lower reflectance. Bands 4-6 represent the infrared colors. In water with a depth greater than 2 meters nearly all infrared light is absorbed by the water while most the blue, green, and, to some degree, red are transmitted. Not very much of the energy is reflected off the surface and the remaining energy that is picked up by the spectroradiometer is due to volumetric reflectance of energy that was previously transmitted through the water.

Figure 2. Radiometric signatures of 12 different features. 

 Standing water is only one of 12 different feature types that I need to record the radiometric signature for. After several minutes of looking around the map I was able to collect all the signatures needed. And because of the different mediums each is made of I was able to see distinct difference in the signatures of the different land cover types (figure 2). When looking at the chart above one can see some of these differences and infer to the reasons why.  Let’s take vegetation for example, it has a higher reflectance of the infrared spectrum than it does the visible. This is because vegetation absorbs much visible light to convert it to energy while it reflects the infrared to prevent damage to proteins within its cells.

Figure 3. Comparing the radiometric signatures of riparian and normal vegetation. 

When interpreting the data it can be difficult to tell differences between feature types that are similar in composition. We can compare riparian (located on a hillsides near rivers) vegetation with regular vegetation (figure 3). All the bands are pretty much the same with the exception of band four; near infrared. Due to the higher water content in the riparian vegetation due to its easy access to water it absorbs slightly more infrared energy than the vegetation that is further from water, meaning it is slightly dryer. The difference of reflectance on band four between the two types is the only significant one in the spectral signature that makes it possible to tell the vegetation types apart.