Techniques of topography Topography

1 techniques of topography

1.1 direct survey
1.2 remote sensing

1.2.1 passive sensor methodologies
1.2.2 photogrammetry
1.2.3 active sensor methodologies

techniques of topography

there variety of approaches studying topography. method(s) use depend on scale , size of area under study, accessibility, , quality of existing surveys.

direct survey

a surveying point in germany

surveying helps determine accurately terrestrial or three-dimensional space position of points , distances , angles between them using leveling instruments such theodolites, dumpy levels , clinometers.

work on 1 of first topographic maps begun in france giovanni domenico cassini, great italian astronomer.

even though remote sensing has sped process of gathering information, , has allowed greater accuracy control on long distances, direct survey still provides basic control points , framework topographic work, whether manual or gis-based.

in areas there has been extensive direct survey , mapping program (most of europe , continental us, example), compiled data forms basis of basic digital elevation datasets such usgs dem data. data must cleaned eliminate discrepancies between surveys, still forms valuable set of information large-scale analysis.

the original american topographic surveys (or british ordnance surveys) involved not recording of relief, identification of landmark features , vegetative land cover.

remote sensing

remote sensing general term geodata collection @ distance subject area.

passive sensor methodologies

besides role in photogrammetry, aerial , satellite imagery can used identify , delineate terrain features , more general land-cover features. have become more , more part of geovisualization, whether maps or gis systems. false-color , non-visible spectra imaging can determine lie of land delineating vegetation , other land-use information more clearly. images can in visible colours , in other spectrum

photogrammetry

photogrammetry measurement technique co-ordinates of points in 3d of object determined measurements made in 2 photographic images (or more) taken starting different positions, different passes of aerial photography flight. in technique, common points identified on each image. line of sight (or ray) can built camera location point on object. intersection of rays (triangulation) determines relative three-dimensional position of point. known control points can used give these relative positions absolute values. more sophisticated algorithms can exploit other information on scene known priori (for example, symmetries in cases allowing rebuilding of three-dimensional co-ordinates starting 1 position of camera).

active sensor methodologies

satellite radar mapping 1 of major techniques of generating digital elevation models (see below). similar techniques applied in bathymetric surveys using sonar determine terrain of ocean floor. in recent years, lidar (light detection , ranging), remote sensing technique uses laser instead of radio waves, has increasingly been employed complex mapping needs such charting canopies , monitoring glaciers.