How to Interpret a Satellite Image

This is an image of fires burning in San Diego County, California taken on 28th October 2003 by satellite UK-DMC.

Starting on 25th October 2003 and later named the Cedar Fire, this human-caused wildfire burned out of control through a large area of Southern California for ten days. Driven by Santa Ana winds, the fire burned 280,278 acres (1,134.2 km2), 2,820 buildings (including 2,232 homes) and killed 15 people including one firefighter before being contained on 3rd November, making it the largest fire in recorded California history up to that time.

This satellite image shows an area approximately 200 km by 150 km in size. The image is put together using the information from the DMC multispectral imager (i.e. the sensor inside the DMC satellite) which takes samples at points on the ground every 32m. The imaging camera responds to three spectral bands (in essence three colours) but, unlike a typical camera which responds to green, red and blue light, the satellite camera detects light in green, red and near-infrared.

Since we cannot see near-infrared we create a false colour composite, i.e. we give a colour to each spectral band. If an object is bright in the green band it will have a blue colour in the image, if it is bright in the red band it will have a green colour in the image and if it is bright in the near-infrared band then it will be red in the image. The resulting satellite image therefore looks distorted in colour in comparison to one taken by an aerial camera.

The most obvious colour distortion is that of vegetation and plant life which is bright in the near infrared band so shows up in a satellite image as red instead of green.

In this image, smoke plumes can be seen coming from red areas. These red areas are the forests of the Cuyamaca region. The blue area to the bottom right of the image is the Salton Sea, a large lake. This is surrounded by a patchwork of much brighter red areas, which are crops growing in irrigated fields.

Beyond the grey-brown San Bernardino Mountains (top right), the bright cream white sandy soils of the Mojave Desert can be seen. At the top in the centre of the image is a curved line of various colours including blue and red. This is the Mojave River with the red areas representing the small areas of vegetation irrigated by the river.