Digital Terrain Model (DTM)
A digital terrain model (DTM) is a mathematical model of a project surface that becomes a three- dimensional representation (3D) of existing and proposed ground surface features. Critical calculations and processes based on the DTM include contouring, cross sections and quantities, drainage models, watersheds, hydraulics, water catchment areas, and cross sections sheets.
A DTM is created through the construction of a Triangulated Irregular Network (TIN) and is based on modeling the terrain surface as a network of triangular facets that are created by simply connecting each data point to its nearest neighboring points. Each data point (having x, y and z coordinates) is the vertices of 2 or more triangles. The advantage of the TIN method is its mathematical simplicity- all DTM calculations are either linear or planar.
The processes and the resulting DTM offer many advantages over a topographic survey. Field data for a DTM is collected in a way that allows TxDOT to use the latest in automated survey technology. Traditional data collection (for a topographic survey) involves taking cross sections, typically every 100 feet, along a horizontal control line or in a grid pattern. Digital terrain modeling has virtually eliminated this practice.
Data points (shots) are taken at every break in elevation with no particular pattern being required. The emphasis is on identifying all features and changes in elevation within project limits. Data is collected using an electronic data collector with an electronic total station. The data points are assigned feature codes, attributes, descriptions, comments, and connectivity linking codes to add intelligence to a point at the time of data entry into data collector.
Information is downloaded from the data collector to a computer, either in the field or later in an office, and is processed using AASHTOWare® Survey Data Management System®(SDMS) software. A SDMS® calculated file is generated for importation into CAiCE™ or GEOPAK Survey™ for further review. The file is then imported into GEOPAK® for project design.
Digital Terrain Model (DTM)
A digital terrain model (DTM) is a mathematical model of a project surface that becomes a three- dimensional representation (3D) of existing and proposed ground surface features. Critical calculations and processes based on the DTM include contouring, cross sections and quantities, drainage models, watersheds, hydraulics, water catchment areas, and cross sections sheets.
A DTM is created through the construction of a Triangulated Irregular Network (TIN) and is based on modeling the terrain surface as a network of triangular facets that are created by simply connecting each data point to its nearest neighboring points. Each data point (having x, y and z coordinates) is the vertices of 2 or more triangles. The advantage of the TIN method is its mathematical simplicity- all DTM calculations are either linear or planar.
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Table 4.4 TSPS Manual of Practice Chart for Tolerances for Conditions
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Condition
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I
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II
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Urban Business, District Urban, Suburban & Industrial
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Rural & Broad Area General Mapping
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Remarks & Formulae
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Error in Traverse Closure
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1:10,000
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1:7500
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System Control Loop
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Unadjusted Level Loop Closure (ft.)
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.04
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.08
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System Control Loop M=Miles
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Secondary Traverse Closure
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1:7500
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1:5000
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Between System Control Points
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Secondary Level Loop Closure (ft.)
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.05
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0.2
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Between System Control Points
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Positional Error of Any Primary Monument (horizontal)
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1:15000
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1:10000
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For monuments used for Triangulation or Radial Surveying in respect to another
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Positional Error of Any Primary Monument (vertical)
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± .03 ft.
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± 0.15 ft.
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For permanent bench marks
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*Contour Interval
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2 ft.
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10 ft.
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Or as needed by the State
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Contour Accuracy
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± ½ Contour Interval
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± ½ Contour Interval
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Positional error of any Photo Control Point (horizontal and/or vertical)
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0.50 ft.
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2 ft.
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Or as recommended by Photogrammetrist
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Location of Improvements, Structures, and Facilities during survey
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± 0.05 ft.
± 0.50 ft.
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± 0.1 ft.
± 1 ft.
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Vertical (inverts, flow lines)
Horizontal
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Plotted location of Improvements, etc.
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± 1/40 in.
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± 1/40 in.
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Symbols may be used for large scale maps indicating Center point
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Scale of maps sufficient to show detail, but no less than
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1” – 200′
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1” – 2000′
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Drawings are to show location of survey monuments and bench marks
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The processes and the resulting DTM offer many advantages over a topographic survey. Field data for a DTM is collected in a way that allows TxDOT to use the latest in automated survey technology. Traditional data collection (for a topographic survey) involves taking cross sections, typically every 100 feet, along a horizontal control line or in a grid pattern. Digital terrain modeling has virtually eliminated this practice.
Data points (shots) are taken at every break in elevation with no particular pattern being required. The emphasis is on identifying all features and changes in elevation within project limits. Data is collected using an electronic data collector with an electronic total station. The data points are assigned feature codes, attributes, descriptions, comments, and connectivity linking codes to add intelligence to a point at the time of data entry into data collector.
Information is downloaded from the data collector to a computer, either in the field or later in an office, and is processed using AASHTOWare® Survey Data Management System®(SDMS) software. A SDMS® calculated file is generated for importation into CAiCE™ or GEOPAK Survey™ for further review. The file is then imported into GEOPAK® for project design.
