Editing Topography (section)

===Remote sensing===
{{Main|Remote sensing}}
[[Remote sensing]] is a general term for geodata collection at a distance from the subject area.

====Passive sensor methodologies====
{{Main|Aerial photography|Satellite imagery}}
Besides their role in photogrammetry, aerial and satellite imagery can be used to identify and delineate terrain features and more general land-cover features. Certainly they have become more and more a part of [[geovisualization]], whether [[map]]s or [[GIS]] systems. False-color and non-visible [[spectrum|spectra]] imaging can also help determine the lie of the land by delineating vegetation and other land-use information more clearly. Images can be in visible colours and in other spectrum.

====Photogrammetry====
{{Main|Photogrammetry}}
Photogrammetry is a measurement technique for which the [[co-ordinates]] of the points in [[Three-dimensional space|3D]] of an object are determined by the measurements made in two [[photographic]] images (or more) taken starting from different positions, usually from different passes of an aerial photography flight. In this technique, the common points are identified on each [[image]]. A line of sight (or [[Ray (optics)|ray]]) can be built from the camera location to the point on the object. It is the intersection of its rays ([[triangulation]]) which determines the relative three-dimensional position of the point. Known control points can be used to give these relative positions absolute values. More sophisticated [[algorithm]]s can exploit other information on the scene known a priori (for example, symmetries in certain cases allowing the rebuilding of three-dimensional co-ordinates starting from one only position of the camera).

====Active sensor methodologies====
Satellite [[RADAR]] mapping is one of the major techniques of generating Digital Elevation Models (see below). Similar techniques are applied in [[bathymetry|bathymetric]] surveys using [[sonar]] to determine the terrain of the ocean floor. In recent years, [[LIDAR]] ('''LI'''ght '''D'''etection '''A'''nd '''R'''anging), a remote sensing technique that uses a laser instead of radio waves, has increasingly been employed for complex mapping needs such as charting canopies and monitoring glaciers.