Detection vs. Resolution
Detection
It is possible to "detect" an object (record its presence or absence) that is much smaller than the resolution required to determine the contents of the image. Similarly, objects can be detected and resolved yet still not be identifiable. For example it is possible to recognize characteristics which fit the pattern of a wheeled vehicle. However until more detail can be resolved it is impossible to identify the type of vehicle as a car or truck.
Spatial Resolution
In a practical sense spatial resolution describes how much detail in an image is visible to the human eye. High-resolution images are sharp and the small details can be seen. However even the best images have a limit to their resolution. If you wear glasses you can experience the effect of reduced resolution simply by taking them off. Without glasses, the view gets fuzzier and small details blend together. The ability to "resolve," or separate, small details is one way of describing what we call spatial resolution.
These images of a Toucan demonstrate the effects
of different spatial resolutions. Each higher level of resolution allows you to
distinguish more detail. At lowest resolution, it doesn't look much like a bird,
even when you know that is the object in the picture. As the spatial resolution
is increased you can see a bird and a beak, but exactly where is the eye? At the
highest resolution, more details become clear, and the image is sharper. In the
last picture you can see additional details around the eye that are fuzzy in the
previous picture.
The spatial resolution of images acquired by a sensor system is
normally expressed in cycles or line pairs per millimeter. It can also be
expressed in terms of the smallest object that can be identified at a fixed
distance. For example, we may describe a satellite imaging system as having
"thirty-meter" resolution. This means that two objects, thirty meters
long or wide, sitting side by side, can be separated (resolved) on a Landsat
image. The same is true of ground based imaging systems. Resolution defines the
distance at which objects can be seen and identified. In the picture above, a
person experienced in viewing different birds may be able to identify the Toucan
from the second picture, but most people would need the resolution of the third
picture.
