Description of TBGIS functions
Generation DEM from various sources
DEM can be generated with the data in following format:
Following interpolation options are provided:
- No data available. Five kinds of hypothetical surfaces can begenerated
- Point file only. Convex boundary is assumed for this option.
- Point file and boundary file.
- List of regularly-distributed height values without header information.
Special treatment is available for data from UNIRAS
- List of regularly-distributed height values with proper header information conformed to the rules set by ASCIIGRID in ARC.
- Point file and boundary coverage.
- .E00 file
- IDW: Exponent for the distant can specified
- SPLINE: with REGULARISED option or TENSION option. The latter is recommend for terrain modeling.
- TREND: order of trend surface can be specified.
- KRIGGING: several semi-variance models can be chosen.
- TIN2LATTICE: creat a TIN first, then, TINLATTICE in ARC is used
- TOPOGRID tool: with this tool, data in contour line coverage, point coverage, polygon coverage can be combined to generate a hydrologically sound DEM. This is one of the enhancement at version 7.x.
DEM quality examination
DEM is the basis of all the spatial analysis followed. Due to the diversity of data sources, lack of proper metadata, scales, as well as interpolation technique involved, proper assessment of DEM quality become a necessity for any terrain-based modelling.
Four methods are designed to aid user to examine the quality of a DEM generated.
- Contour: graphic display of contour lines. Contour lines from original topographic map, if available, can be overlapped for comparison.
- Cumulative frequency distribution: graphic display of cumulative frequency distribution curves. Multiple grids and items in an info file can be compared within one coordinate system. Optionally, the results can be saved to an info file. This method has been widely used in the assessment of scale effect.
- Control points: Points with known height values in a plain ASCII file can be used as bench marks. Maximum underestimated, maximum overestimated, mean underestimated, mean overestimated, and RMS are reported.
- Blue line method: After examing the histogram of flow accumulation, user can specify a threshold value to creat linear coverage of potential drainage system. The correct location and composition of drainage system is essential for realistic representation of water movement on the surface. So, this method may be very useful for hydrologist.
Calculation of terrain-controlled erosion potential
Estimation of process-based erosion potential is carried out by:
- first extracting relevant terrain attributes with Arc/Info GRID functions, using existing grids, or inputting
result from TAPES-G. Following terrain attributes can be used as inputs: slope gradient (in percent, degree, sine value, tangetial value), flow direction (D8 only), aspect, upslope contributing area, plan curvature, profile curvature, flow width, and flow length.
- Then, calculating terrain-controlled erosion potential.
- Finally, saving terrain attributes grids if necessary.
Estimation of relative erosion and deposition
Neighbourhood function in Arc/Info Grid is used to identify the neighbour cells
of a processing cell. Erosion potential of the central cell is taken as the
actual sediment output and erosion potentials of neighbour cells are taken as
the possible inputs. Based on the principle of mass conservation and assumption
of transport-limited condition, the difference between input and output is
regard as the relative erosion and deposition value for the processing cell.
Three mechanism are available to decide the actual input from neighbour cells.
- D8 method
- This method is based on the flow direction given by Arc/Info GRID.
It means that the sediment of a cell can only be moved to one of its
eight neighbour cells. This method is used in EROS of TAPES-G.
- Slope method
- This method defines relative erosion and deposition value as the change of erosion potential along the flow path. Aspect is used as the indication of flow direction. A simple
differential mechanism is used to derive the change of erosion potential along the X-axis and Y-axis direction.
This method was originally proposed by Mitasova and applied on the GRASS GIS
- Multiple paths method
- This method is based on the relative heights among neighbour cells and
processing cell. Erosion potential of a cell become the input of all neighbour
cells which has lower heights. The gradient between processing cell and a
neighbour cell determined the proportion of erosion potential this neighbour
cell will receive.
Transformation of relative erosion and deposition to caesium-137 inventory at sampling time
For most of the cases, you do not have to use this function unless you are
really keen on the combination of erosion and deposition modeling with caesium-137 techniques.
Caesium-137 inventory has been shown to be a good indicator of net erosion and
deposition patterns. For validating the prediction of erosion and deposition patterns from TBGIS, it is necessary to calibrate the relative erosion and deposition values to caesium-137 inventory at sampling time. Dr Qingping He's method is implemented for this purpose. A station file containing the actual or relative annual air deposition of
caesium-137, local reference caesium-137 inventory, avearge tillage depth are
required for this operation.
Data manipulation, analysis, visualization, and export
Terrain modeling is a multiple-step, iteriative processes. Tools are provided for manipulating, viewing, analysing, and exporting original data, intermediate results, final results.
Tools are available to generate info file from a plain ASCII file; generate point, line, or single polygon coverage from plain ASCII file; and convert TIN to a grid.
- list, rename, copy, or delete existing grids, stacks, covers, tins, info files, system files, workspaces.
- view ASCII data file, check items in an info file, scroll through an info table, graphic display of items in an info file, overlay of point, line, polygon coverages.
- map view of grid and its derived hydrologic terrain attributes. It can be positive or negetive image(coverages can be overlayed on the top), map of flow direction (D8), labelled basin delineation, classified or unclassfied color shading.
- stack view of multiple grids
- point sampling: cell values or interpolated values for points with known locations can be sampled. Those points can be in a file, point coverage, or from interactive inputs.
- classic statistics: summary information for a grid or an item in an info file; linear regression among grids; correlation between two grids or two items in an info file.
- spatial statistics: calculate Moran or Geary index for a grid.
- grid, coverage, info, tin can be exported in .E00 format
- all or selected items in an info file can be downloaded to an ASCII file
- grid can also be exported to an ASCII file or in a format suitable for UNIRAS 3-D or 4-D mapping.