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Statistical surfaces: DEM’s

Statistical surfaces: DEM’s. Geog 4103, March 22. Real world phenomena represented as: . DISCRETE: homogeneous or spatially averaged units, e.g. subwatersheds, counties, polygons VECTOR FIELDS: discretized as grid cells or meshes RASTER. What are surfaces ?.

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Statistical surfaces: DEM’s

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  1. Statistical surfaces:DEM’s Geog 4103, March 22

  2. Real world phenomena represented as: • DISCRETE: • homogeneous or spatially averaged units, e.g. subwatersheds, counties, polygons • VECTOR • FIELDS: • discretized as grid cells or meshes • RASTER

  3. What are surfaces? • Features that contain Z values distributed throughout area defines by (x,y) coordinate pairs • Z values can be any measurable phenomena that varies across space (temperature, elevation, precipitation, etc…) • called “field” like, or continuous data

  4. What is a field? • a conceptual model of geographic variation • at every point in the frame (x,y) there exists a single value of a variable Z • e.g. a field of temperature • e.g. a field of land surface elevation • the variable may be measured on any scale • temperature - degrees Celsius • elevation - meters above sea level

  5. Field data are continuous • a field is spatially continuous by definition • values exist everywhere

  6. Representation of field phenomena A) CELLS B) REGULARLY SPACED POINTS C) IRREGULARLY SPACED POINTS D) CONTOURS E) POLYGONS F) TINs- Triangulated Irregular Network

  7. - used for continuous data Isarithmic Mapping Data measured at points Derived data

  8. Spatial sampling e.g. elevation • Regular lattice • restricted to • X,Y locations • Irregular lattice • not restricted • based on knowledge about how • smooth/rugged the surface is

  9. Two methods of representing a surface inside a computer • Vector surfaces: • TIN’s (Triangulated Irregular Network) • Raster surfaces: • DEM (Digital Elevation Model)

  10. RASTER vs. VECTOR DIGITAL ELEVATION MODEL

  11. Triangulated Irregular Network (TIN)

  12. Triangulated Irregular Network (TIN) • continuous mesh of triangles. • -triangles vary in size • based on roughness/complexity • of terrain. • - Large vs. small triangles

  13. Raster Data Model • A raster representation is composed of a series of layers, each with a theme • Typically used to represent ‘field-like’ geographic phenomena

  14. Raster Grid • but most common raster is composed of squares, called grid cells • grid cells are analogous to pixels in remote sensing images and computer graphics

  15. 1 1 1 2 3 1 1 1 2 3 2 2 2 2 2 4 4 4 2 3 4 4 4 2 3 Raster Resolution Spatial resolution = the distance that one side of a grid cell represents on the ground = grid cell resolution The higher the resolution (smaller the grid cell), the higher the precision, but the greater the cost in data storage

  16. The DEM / DTM • Digital elevation models = a way of representing surfaces. • Quantitative model of a topographic surface in digital form. • data sets are continuous surfaces.

  17. Elevation data • Source of DEMs and TINs • Process of interpolation - creating continuous data from point data

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