Geographic data come in many types, from many different sources and captured using many techniques; they are collected, sold, and distributed by a wide array of public and private entities.
In general, we can divide the collection of geographic data into two main types:
- Directly collected data
- Remotely sensed data
Directly collected data are generated at the source of the phenomena being measured. Examples of directly collected data include measurements such as temperature readings at specific weather stations, elevations recorded by visiting the location of interest, or the position of a grizzly bear equipped with a GPS-enabled collar. Also included here are data derived through survey (e.g., the census) or observation (e.g., Audubon Christmas bird count).
Remotely sensed data are measured from remote distances without any direct contact with the phenomena or a need to visit the locations of interest (although directly collected “ground truth” data are often used to support accurate interpretation of the remotely sensed data; this is a topic we will pick up in Chapter 8). Satellite images, sonar readings, and radar are all forms of remotely sensed data.
For each type of data, there is a range of important issues about collection and processing that have an impact on how reliable and useful the data are. The federal agencies that collect and distribute geographic data and the standards by which they operate will be covered in Chapter 8.