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GIS

Productivity Corner: Dig Up Accurate Water Data with TRex

6 Sep, 2006 By: Andres Gutierrez

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Extract elevation data for water modeling using Bentley WaterCAD.

The latest version of Bentley WaterCAD, V8 XM Edition, offers a new elevation extraction utility that helps engineers build water models easily by extracting node elevation data from several types of digital elevation models.

Obtaining elevation data for input into a water distribution model can be a time-consuming and expensive process. In some cases accurate elevation data may be critical to the model accuracy, while in other cases it can represent unnecessary resource expenditure.

Elevation Data for Hydraulic Modeling
In order to decide on the appropriate level of quality of elevation data to be gathered, it's important to understand how a model uses this data. For model nodes, elevation data is not directly used in solving the network hydraulic equations. Instead, the models solve for HGL (hydraulic grade line). Once the HGL is calculated, the elevations are then used to determine pressure values.

If the modeler is only interested in calculating flows, velocities and HGL values, then node elevations are not necessary. Now, if the modeler specifies pump controls or pressure valve settings in pressure units, then accurate elevation values are required at each key node.

Accuracy and Precision
How accurate must the elevation data be? The answer depends on the accuracy desired in pressure calculations vs. the amount of labor and cost allotted for data collection. For example, the HGL calculated by the model is significantly more precise than any of the elevation data. Because 2.31 feet of elevation translates into 1psi of pressure (for water), calculating pressure to 1psi precision requires elevation data that's accurate to roughly 2 feet. Elevation data that's accurate to the nearest 10 feet will result in pressure that is accurate to roughly 4psi.

The lack of precision in elevation data (and pressure results) also leads to questions regarding water distribution design. If design criteria state that pressure must exceed 20psi and the model gives a pressure of 21psi (+/-4) or 19psi (+/-4), the engineer must decide if this design is acceptable.

Obtaining Elevation Data
When building the large models that are used today, collecting elevation data is often a time-consuming process. A good modeler wants to devote the appropriate level of effort to data collection that will yield the desired accuracy at a minimum cost. Some of the data collection options are:

• USGS topographic maps
• Surveying from known benchmarks
• DEMs (digital elevation models)
• SDTS digital elevation models
• Digital ortho-rectified photogrammetry
• Contour maps (contour shapefiles or CAD)
• As-built plans
• GPS (global positioning systems)

Different elevation data sources have different precisions and the modeler needs to take this into account, balancing the need for accuracy with data acquisition costs. For example, DEMs (available from the USGS) are available at several scales. For water distribution, it's best to use the 30-meter DEMs that include an elevation value for each 900 square meters.

The topographic characteristics of the network are also important. DEMs produce the best accuracy in areas that are relatively flat with smooth slopes but have poorer accuracy in areas with large, abrupt changes in elevation, such as cliffs and road cuts.

Calibration Nodes
An elevation accuracy of 5 feet is adequate for most nodes. However, for nodes to be used for model calibration, a higher level of accuracy is desirable. Consider a situation where both the model and the actual system have exactly the same HGL of 800 feet at a node. The elevation of the ground is 661.2 feet while the elevation of the pressure gage used in calibration is 667.1 feet. The model would predict a pressure of 60.1 psi while the gage would read 57.5psi even though the model is correct.

A similar error could occur in the opposite direction with an incorrect pressure appearing accurate because an incorrect elevation is used. This is one reason why you should calibrate models by comparing modeled and observed HGL values and not pressures.

TRex Terrain Extractor
The TRex (Terrain Extractor) module, new for WaterCAD V8 XM Edition, is designed to expedite the elevation assignment process by automatically assigning elevations to the model features according to the elevation data stored within different types of DEMs.

TRex quickly and easily assigns elevations to any or all of the nodes in the water distribution model. Data input for TRex consists of:

1. Specify the elevation data source (DEM):

DXF contours

Land XML

Model spot elevations

GIS shapefiles

2. Specify the measurement unit associated with the DEM:

Feet

Meters

Any other appropriate unit

3. Select the model features to which elevations should be applied:

All model nodes (tanks, reservoirs, junctions, pumps, hydrants, valves, etc.)

Graphical selection of node elements

Pre-defined selection set of node elements

TRex then interpolates an elevation value for each specific point occupied by a model feature. The final step of the wizard displays a list of all of the features to which an elevation was applied, along with the elevation values for those features.

You then can apply these elevation values to a new physical properties alternative or an existing one. In some cases, you might have more accurate information for some nodes, such as survey elevation from a pump station. In those cases, you should create the elevation data using DEM data and manually overwrite the more accurate data for those nodes.

The TRex module in WaterCAD V8 XM Edition simplifies the process of applying accurate elevation data to water distribution models. As was shown, accurate elevation data is vital when accurate pressure calculations and/or pressure-based controls are required for the water distribution model in question. All elevation data for even large distribution networks can be applied by completing a few steps.

Please click here or more information about Bentley WaterCAD and the TRex Module.