A regular map is used to find out where things are. The difference between mapping that provides geographical reference and mapping that provides the quantitative, three-dimensional information required by serious technical studies lies in the difference between a Digital Terrain Model (DTM) map and other maps. DTM maps are not replacing other types of maps. Still, they are essential to a variety of technical studies that support viable development proposals, such as environmental impact assessments, drainage engineering, and flood risk analysis.
What a DTM actually contains
A Digital Terrain Model is a mathematically accurate representation of the surface of the ground as a grid of elevation points that cover the study area. The points in the grid are at a certain elevation above the datum, and the number of points dictates the resolution of the model. The complete three-dimensional form of the terrain can be determined from this set of values such as slope angles, aspect directions, drainage catchments, elevation profiles along any given line, and volume between existing and proposed surfaces. All of this information cannot be obtained from a standard plan, no matter how detailed the plan is in other aspects, making a DTM map essential for planning developments.
Drainage design and the catchment question
Without understanding of the natural flow of water across and off the development site, drainage engineers cannot design a drainage system. DTM data can be used to automatically delineate drainage catchments, identify natural flow paths and estimate runoff volumes for pipe sizing, attenuation design, and placement of drainage infrastructure. A drainage scheme designed without DTM data will use field observation and professional judgment to fill in the gaps that quantitative terrain analysis would provide, adding uncertainty to calculations that have consequences if they are incorrect, namely flooding in just the situations drainage design is meant to prevent.
Flood risk assessment and elevation accuracy
The accuracy of ground elevations within and around the study area is important for flood risk assessments as the difference between a ground elevation within and above a modelled flood extent can be several centimetres, not metres. DTM data at an appropriate resolution is sufficient for the elevation accuracy needed to establish flood risk assessment, so that the risk can be determined and the relationship between the proposed finished floor levels and the modelled flood levels can be established. The contour data from standard mapping will exhibit unacceptable interpolation errors when used for this purpose, and the assessment results will have direct planning and insurance implications.
Cut and fill calculations in development
The volume of material to be excavated or imported to reach the desired levels in a development project that involves earthworks is always calculated. DTM data can be used to determine the ground surface level and, when used in conjunction with the proposed surface, the cut and fill volumes. This calculation is directly related to the accuracy of the earthworks cost estimate, the waste management planning for the disposal of surplus material, and the determination of the proportionality of the earthworks to the actual development proposal. Estimates of volume from contour plans are less quantitatively reliable than those from DTM data.
Environmental impact assessment requirements
For major development projects, quantitative assessments of the site and its environment are conducted across several environmental parameters as part of an environmental impact assessment. Landscape and visual impact assessment involves modelling the viewsheds to determine which portions of the landscape are theoretically visible to the proposed development and what mitigation will be needed to manage the visual impact. Noise modelling is a tool that is used to determine the impact of topography on the propagation of noise from the development. Shadow and daylight analysis is a method for evaluating the interaction between development and solar geometry, based on three-dimensional terrain information. The accuracy of these analyses depends on the accuracy of the DTM data, which in turn affects the credibility of the analyses’ conclusions.
Why resolution matters
DTM data comes in various resolutions, each suitable for different applications. One-metre resolution DTM may be sufficient for a regional flood risk screening study. Engineering design for a particular drainage structure may require a resolution of 25cm or less to meet the design’s precision requirements. Choosing DTM data at a resolution too low for the application will yield an analysis that may appear accurate but actually contains interpolation error at the scale of the application. The resolution of each application is part of the specification of the right data product.
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