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Development of Drainage Tools for Facilitating the Creation of Topologically-Sound, Unidirectional Flow Tile Networks

Anamelechi Falasy1,*, Richard Andrew Cooke1

Published in Journal of the ASABE 67(2): 337-348 (doi: 10.13031/ja.15517). Copyright 2024 American Society of Agricultural and Biological Engineers.

1    Agricultural and Biological Engineering, University of Illinois, Urbana, Illinois, USA.

*    Correspondence: falasy567anamelechi@gmail.com, falasya2@illinois.edu

Submitted for review on 30 December 2022 as manuscript number NRES 15517; approved for publication as a Research Article and as part of the “Advances in Drainage: Selected Works from the 11th International Drainage Symposium” Collection by Associate Editor Dr. Mohamed Youssef and Community Editor Dr. Zhiming Qi of the Natural Resources & Environmental Systems Community of ASABE on 8 December 2023.

Highlights

Abstract. An important step in designing a drainage system is laying out the network. Successful installation and sizing of the system require that the network be topologically sound and unidirectional. Online digitization of drainage systems does not always result in networks that meet these criteria. Tile lines may not be consistently digitized from upstream to downstream, and there may be disconnected segments, dangles, and overshoots. We have developed a set of open-source tools (plugins) in QGIS that convert a series of laterals and mains digitized on a map into a topologically-sound drainage network to facilitate both burying and sizing the drainage lines. Steps include reversing node numbering, if necessary, to ensure that tile nodes are numbered from upstream to downstream, connecting the ends of tile lines that are within a certain tolerance of each other, correcting overshoots, and setting an order for burying or calculating cumulative flow in line segments. An application example is presented using a proposed tile network layout at the university farm to demonstrate the capacity of these new tools to fix topologically disconnected tile lines and generate sound tile networks that can be successfully buried and sized. These easy-to-use tools are compatible with all versions of QGIS3 and may be freely downloaded or installed from the official QGIS Plugin Repository.

Keywords.Line Geometry, Plugins, QGIS Software, Tile Layout Routines, Tile Nodes and Lines.

  1. Subsurface drainage systems have been used to improve crop yield and the timeliness of field operations in many regions of the world. In the US, there are likely 22.4 million hectares (55.4 million acres) of subsurface drained land, mostly in the Midwest (Frankenberger et al., 2022). For most of human history, the objective of drainage has been to optimize yield and the timeliness of field operations. However, for the past 50 years, there has been an increasing emphasis on optimizing drainage for environmental or water harvesting benefits. This new emphasis has led to the development of in-field conservation drainage practices such as drainage water management, also called controlled drainage, and depth/spacing modifications, as well as edge-of-field practices such as bioreactors and saturated buffers. In-field practices, as well as the design of drainage systems for subirrigation or drainage water recycling, often require new approaches to drainage layout. Another recent development that has the potential to affect drainage system layout and pipe sizing is the disaggregation of what was formally called the drainage coefficient into three coefficients: the Kirkham Coefficient (KC), the Drainage (or Design) Intensity (DI), and the Drainage Coefficient (DC) (Skaggs, 2017). Drainage systems can be optimized by uncoupling the flowrates used for determining lateral depth and spacing, lateral sizing, and the sizing of sub-mains and mains.

Drainage system design can be simplified using computer-aided design packages. Currently, there are at least three proprietary design packages used by drainage contractors: The Trimble WM-Subsurface Farm Drainage Software ensures the optimal placement of tile and surface drains in both surface and sub-surface drainage water management projects, helping to drain fields adequately and increase crop yields (Trimble, 2017). Some key features of the software are its ability to tie laterals to mains, create parallel lateral spacings, clip drainage lines, and verify that the pipe network will successfully drain to the main outlet using the drainage line creation tool. The AGPS-Pipe Pro aids in laying drainage pipe or tile (CooksAGPS, 2023). The program captures the topography of a tile line, uses the information to calculate the elevation and grade of the pipe and controls the plow blade in laying the pipe. The TileLogic MC program aids mostly in the drainage installation process and for field survey and mapping but is also capable of performing virtual burial and sizing calculations for large projects with multiple drainage networks (Whitebarntech, 2013). For the most part, these packages are not affordable for research and education purposes. Other more affordable or free software packages suitable for research, such as LANDRAIN (Sands et al., 1985), SUBDRAIN (Steenhuis et al., 1987), MACDRAIN (Kok and Tremblay, 1988), and DrainCAD (Feyen et al., 1990), predate the widespread use of geographic information systems (GIS) for drainage design and Microsoft Windows. None of the design software packages mentioned above used different coefficients for different aspects of the design process.

The emergence of advanced technologies, such as GIS and high-resolution remote-sensed elevation data, and the uncoupling of design coefficients have opened new possibilities for optimizing the design and operation of drainage systems.

A schematic of the drainage design process leading to installation is shown in figure 1. The steps include (1) collecting background information on soils, topography, and crops; (2) determining if drainage is needed; (3) determining if there is a suitable outlet of adequate size; (4) selecting a design intensity and a suitable drain depth and spacing; (5) laying out the laterals and mains for the drainage system; (6) building the pipe into a topologically-sound and unidirectional network; and (7) determining suitable depths and grades and sizing the drainage pipes. We have developed a set of GIS-based tools collectively called the “Illini Drainage Tools” (IDTs), to facilitate the drainage design process. These tools are divided into three groups. The first group aids with the layout and digitization procedure, the second set with network development, and the third with pipe burying (determination of grades and depths) and sizing. In this study, we present the second set of tools, that is, the tools used to create topologically sound, unidirectional, subsurface drainage networks.

Figure 1. Drainage design process flowchart. This manuscript focuses on the highlighted step: building pipes into topologically sound and unidirectional network.

In GIS-based design of drainage systems, the tile lines are typically laid on a map prior to using the topography to set pipe slopes (Shedekar and Brown, 2018). Each pipe is then sized based on the seepage rate into it and the cumulative flow from upstream pipes (Kalita et al., 2007). However, the digitization process in GIS may result in discontinuities (Chung et al., 1995). A network that appears to be continuous on screen may have unconnected segments or overlapping segments when viewed in greater detail. Such discontinuities and inconsistencies need to be fixed to determine flow in the network (Berkowitz, 2002).

The digitization of map-features in GIS basically involves the preparation of a vector layer by tracing its features in the form of points, lines, or polygons. In one such GIS, Quantum Geographic Information Systems (QGIS), for example, this is done by first initializing the “Toggle Editing” mode for a highlighted layer and then using (Ctrl +.) to activate the crosshair, which allows for efficient precision, pinpointing, and tracing of map features. This means that, quite often, the digitizer plays a major role in managing the accuracy of the digitized features. Thus, for every digitized map feature, its accuracy is ultimately dependent on the number of errors that occur during the digitization process. While digitizing (adding lines) for subsurface drainage layouts, line segments are expected to be connected to each other. Each line segment has both a starting and an ending node that sets the topological structure of the line feature. As water only flows downstream, subsurface drainage networks are unidirectional. For the pipe segments in these networks, the start nodes should all be upstream, or they should all be downstream. However, they are not all digitized in the same direction in practice. Additionally, tile lines may have discontinuities in the layout structure. Positional error errors can occur during the digitization process is positional error, preventing reliable network analysis within a GIS (Feuchtwanger, 1993). Figure 2 shows some of the positional errors and discontinuities that can occur in a typical drainage system layout. The detection and removal of such discontinuities of spatial data can be analyzed in terms of their geometry and topology (Mara et al., 2010; Peacock et al., 2016).

In QGIS, every basic tool and add-in routine (plugin) is designed to address a specific task based on the developer’s needs. For example, the NetworkGT Plugin Toolbox comprises of a set of tools developed for the geometric and topological analysis of fracture networks (Nyberg et al., 2018). This plugin identifies attempts to repair topology errors for line features. However, it does not fix discontinuities in nodes where more than two lines intersect (fig. 2a). Another plugin, the WaterNetAnalyzer (Schilling, 2021), can be used to create networks but does not fix the positional errors resulting from digitization. Significantly, QGIS includes a built-in plugin called 'Topology Checker,' which identifies and flags topology errors in a vector layer based on user-specified rules. Thus, a user must manually select the rules to be applied in Topology Checker. Once errors are identified, the user must manually fix each individual problem, regardless of how many there are.

We have developed a set of routines to convert a series of digitized drainage laterals and mains into a topologically-sound drainage network and to set the calculation order of line segments in the network to facilitate both burying and sizing the drainage lines. These new free tools incorporate the functionality of the Topology Checker plugin and, in addition, identify and fix errors with a single click. When a user populates the simple and user-friendly input boxes, the tools identify and correct digitization errors, regardless of the type of line discontinuities associated with each node. The purpose of these tools is to represent the network as a series of geometrically aligned links and nodes, to set the calculation order of line segments, and to determine the cumulative flow in line segments in the network to facilitate both burying and sizing the drainage lines.

In this study, we demonstrate their capabilities by using the new tools to build a well-structured and comprehensive unidirectional drainage flow layout network that is topologically sound for a field on the South Farm of the University of Illinois in Urbana-Champaign. We also use the QGIS Topology Checker to analyze the topology of layouts produced for a field by the tools and by commercial drainage software.

(a)(b)(c)
Figure 2. Drainage system with positional errors (a) fracture, (b) overshoot, and (c) dangle.

Materials and Methods

design and Development in QGIS

The tools were developed in the Quantum Geographical Information System (QGIS), a free and open-source GIS software platform. QGIS allows for the development of new add-in routines (plugins) in the Python 3.8 programming language (http://python.org) and provides online support through forums, tutorials, and online documentation (http://www.qgis.org). QGIS uses these plugins or extensions to add new functionality to QGIS without altering the main software program itself. These software components enhance the capability of the main program. QGIS is in the public domain and is available in 48 languages. The source code is freely available, there is a large community of developers, and it has been implemented in several computer operating systems, including Mac OS X, Linux, Unix, and Microsoft Windows. Many applied scientists have used the QGIS platform to develop plugins specifically tailored to their field of interest.

Building on network functionality in the QGIS processing toolbox, we have developed five tools that together address discontinuities in vector line networks and perform other tasks associated with building a well-structured and comprehensive drainage network layout system that is topologically sound and unidirectional:

  1. Hydraulic Network Fixer [E]
  2. Network Flow-Path Generator [F]
  3. Tile Network Ordering [G]
  4. Network Elevation Exports [H]
  5. Network Flow Lengths [I]

To build these new tools, Plugin Builder 3.2.1 (GeoApt, 2022) and Plugin Reloader 0.9.1 (Jurgiel, 2022) were used. Qt Creator 5.0.2 (QT Company, 2022) was used to develop the graphical user interface (GUI), and Python 3.8 with the QGIS3/Python standard libraries (gdal, math, numpy, datetime, and os) was used for the associated functionality. Detailed installation instructions and the source code for the tools can be found on: github.com/FVW-Services/Illini-Drainage-Tools/blob/main/README.md.

Application and Testing in QGIS

The new tools were tested in two phases: first, the tools were used to develop a drainage system layout for a field at the South Farm of the University of Illinois in Urbana-Champaign (fig. 3). In phase two, we compared the results from a drainage design layout created with commercial drainage software (fig. 4) and the layout created using the new tools, both in QGIS, to illustrate the novelty of the new tools. Additionally, raster LiDAR data from the Illinois Height Modernization database (https://clearinghouse.isgs.illinois.edu/data/elevation/illinois-height-modernization-ilhmp) was also used for extracting elevation details for the fields. Data used for testing these tools may be accessed on the Illinois Drainage Guide (https://publish.illinois.edu/illinoisdrainageguide/).

Figure 3. Google arial view of the university test field with subsurface drainage layout.
Figure 4. Subsurface drainage layout created using commercial design software.

Creating Topologically -Sound  Line Networks in QGIS

Hydraulic Network Fixer

The first stage in the process leading to the creation of a topologically-sound network involves the use of the Hydraulic Network Fixer tool that generates geometrically aligned nodes from digitized vector lines. The required input for this tool is a shapefile with a set of digitized vector lines, as shown in figures 3 and 4. The Hydraulic Network Fixer tool first identifies and interprets the digitized layout network into the QGIS data architecture, examines the line network structure, and identifies all possible positional errors associated with the line network. The interface for the Hydraulic Network Fixer tool is shown in figure 5. The GUI for this and all the other tools in the IDTs is comprised of a main window with two different input sections (A, B) and a usage information window (C). Section A contains the input parameters that are essential for the tools to work, while Section B defines the output data files resulting from running the tools.

Depending on the layout of the system, the Hydraulic Network Fixer tool performs the following sequence of tasks:

  1. Extends the endpoint of each line segment to connect with another line, thus forming a node from fractured lines.
  2. Dissolves attributes to aggregate all fracture lines features into a single feature line layer.
  3. Extracts nodes at the endpoints and intersections, if any, of each line segment.
  4. Generates location coordinates for these newly generated points.
  5. Snaps the endpoint nodes of a line segment to another node within a specified tolerance.
  6. Creates a new line layer by connecting the features from the nodes together.
  7. Fix issues related to line geometry, such as adding geometry attributes and removing line features with null entries in the attributes.
  8. Reproduces the line segments in the original layout.

When applied, this application tool recreates the network layout after fixing all positional errors from digitization, thus exploding the system layout into connected line segments that are presumed to be sound topologically.

Line Network Validation and Testing in QGIS

It is usually difficult to visually determine if a tile layout is well connected geometrically. Thus, the Network Flow-Path Generator tool, from the IDTs plugin, was used to demonstrate the development of a topologically sound subsurface network layout system. This Network Flow-Path Generator tool uses the output generated from the Hydraulic Network Fixer tool to reorient the line segments so that the beginning and end nodes of each is consistently upstream and downstream, respectively, and identifies line segments immediately upstream and downstream of each line segment. Like the WaterNetAnalyzer plugin developed by Schilling (2021), this tool requires the specification of the outlet line segment. The outlet line segment is easily selected on the map canvas from the corrected network layer using the “Select Feature by Area or Single Click” tool in QGIS. This tool automatically sets and corrects the flow direction of a network after the user has identified the outlet in the network layout.

Figure 5. Graphical user interface of the hydraulic network fixer tool: (A) input parameters, (B) output datafiles, and (C) information window.

Creating Unidirectional Flow  Tile Networks in QGIS

In QGIS, establishing unidirectional labeling of nodes in line segments makes it possible to establish a flow path, upstream to downstream, in the entire network of line segments by calculating its order and flow strength. Three tools from the IDTs plugin, (a) Tile Network Ordering, (b) Network Elevation Exports, and (c) Network Flow Lengths, were used to create such a network.

Tile Network Ordering

The Tile Network Ordering tool was developed to determine the flow line path in the tile layout. Using the results from the Network Flow-Path Generator tool, Tile Network Ordering uses the field IDs of corresponding line segments to determine the flow direction in the tile network based on the connecting nodes in the layout. Moreover, by applying the top down principle of Strahler Stream Order (Pradhan et al., 2014; Strahler, 1957), as shown in figure 6, the flow order for each pipe in the tile network is determined. The number 1 stands for the first-order, which represents the smallest and most numerous pipes in the network, in this case, the lateral lines. When two pipes of the same order are joined together, the pipe immediately downstream is classified as a higher order pipe. Thus, when two first-order pipes are joined together, the pipe immediately downstream is a second-order pipe, which is represented by the number 2. Order 2 and above are used for the sub-mains and/or mains in the tile network. However, when two pipes of different orders are joined together, the pipe immediately downstream is classified as the same order as the higher order of the two upstream pipes. This same pattern continues until the last order for the outlet pipe in the network is specified.

Connecting the ends of tile lines that are within a certain tolerance of each other provides a hierarchical system within the network layout (Gleyzer et al., 2004; Langen and Griffith, 2013). This ordering of the line segments in tile network layout reflects the flow strength in the drainage system and forms the basis of important hydrographical indicators of its structure, such as its drainage density and frequency.

Network Elevation Exports

The Network Elevation Exports tooluses the output from the Tile Network Ordering tool to extract elevation points for each line segment by draping a digital elevation model (DEM) over the drainage network and adding four additional fields, the starting elevation, the endpoint elevation, the true length, and the slope of each line segment, to the attribute table for the tile network.

Network Flow Lengths

The Network Flow Lengths tool was developed to determine the cumulative flow lengths for all connected adjoining line segments upstream to downstream in the tile layout network, similar to Nyberg et al. (2015), Schilling (2021), and Zhang et al. (2008). The tool uses the output from the Network Elevation Exports.

Figure 6. Strahler Stream Ordering of a drainage system network.

The five tools from the IDTs presented in figure 6 can be installed from the official QGIS plugin repository. Once executed, the tools provide their respective output layers. The main execution source codes for the tools are primarily contained in the Python file as: geometrically_fixed_algorithm.py, network_generator_algorithm.py, network_flow_?ordering_algorithm.py, network_elevation_algorithm.py, and flow_lengths_algorithm.py. These files create and control the GUIs on the front-end, while the interactive GUIs in QT designer mode displays the function keys of the tools. Instructions on how to use these tools and test data are provided in the Illinois Drainage Guide: publish.illinois.edu/illinoisdrainageguide/illini-drainage-tools.

Results

The results from the application example as shown in figure 7 illustrate the capacity of the tools to successfully identify and fix the positional errors associated with the digitization of line segments, as shown in figure 2. In addition, the result from using the QGIS “Topology Checker” plugin on a digitized vector line shapefile created using commercial drainage software further demonstrates the relevance of the new tools in filling the gap of creating sound topology in vector line layers for non-proprietary software packages.

The screen display from the Hydraulic Network Fixer tool is shown in figure 7. Shown is the recreated layout. The result illustrates the tool’s capacity for creating a topologically sound drainage layout network. Discontinuities like fractures, overshoots, and dangles in the network have all been corrected, unlike in figure 2. Moreover, at the end of running all five of these tools consecutively, a well comprehensive drainage layout system is built with outputs as follows:

Figure 8 shows a sample log report screen after running the Network Flow-Path Generator tool, with the zero number of unconnected segments underlined after this correction. The expression “Network generated with zero (0) unconnected segments” shows that no topology errors exist in the hydraulic layout. It also indicates if the routine was successfully run, the execution time, and the name and location of any output files produced. Similar reports are produced for all the tools.

Figure 9 shows the screen display and attribute table results of using the QGIS Topology Checker plugin to test for invalid geometries and positional errors on (a) the digitized line layout created with commercial drainage software and (b) a similar network layout created with the new tools in QGIS. The attribute table in figure 9a shows the presence of dangles and other discontinuities in the layout created with the commercial software; hence, no feature ID is generated for the line layout. On the other hand, the attribute table in figure 9b shows the result of a topology test on a similar layout created by the new tools, with the corresponding line feature ID generated. Generally, while the QGIS Topology Checker plugin examines a vector layer for its network topology properties, it also highlights unconnected endpoints as errors in a network, hence the 71 errors in figure 9a and the 34 errors in the attribute table of figure 9b.

On further testing using the “Network Flow-Path Generator” tool, figure 10 illustrates a difference in the topological structures between the original digitized tile layout and the corrected network. When run without first fixing the positional errors of digitization, the drainage layout created does not reorient the line segments properly, upstream to downstream, with the flow direction in the network not set correctly (fig. 10a). The attribute tables show the transformation from a series of unconnected lines (fig. 10a) to a topographically-sound network (fig. 10b). The outputs table, as illustrated in figure 11b, thus serves as a recipe for determining the direction of flow from the upstream inlet to the downstream of the line segment, as well as in determining the flow order for each line segment in the tile layout. The difference herein is that while the other software created a series of unconnected lines, the new IDT routines created a connected network that is both topologically sound and unidirectional.

An output attribute table and the corresponding screen display after running the Tile Network Ordering tool are shown in figure 11. By applying the recursive rules to the “TILE_TO” field in figure 11a, the “TILE_FLOW,” which indicates the flow line path for the tile network, is determined. The tiles are ordered from upstream to downstream in the FLOW_LINE column, used for flow calculations, and from downstream to upstream in the TILE_FLOW column (green), used for calculating pipe slopes (fig. 11b). The numbering in figure 12b shows the sequence the sizing of pipes follows in the system, upstream to downstream (1-48).

Figure 7. A newly generated drainage layout that is connected and topologically-sound.
Figure 8. Log screen report after running the network flow-path generator tool.

Figure 12 also shows the attribute table and the corresponding screen display after running the Tile Network Ordering tool. The field “TILE_ORDER” in figure 12a highlights the hierarchical structure within the tile network layout. This ordering of the line segments of the tile network layout reflects their individual flow strengths in the drainage system. This means that the ability to differentially size each group of pipes based on their orders can be incorporated as illustrated in figure 12b.

Figure 13 shows the attribute table produced after running both the Network Elevation Exports and Network Flow Lengths tools sequentially. The former tool adds columns for length and surface slope of each pipe segment, while the latter adds a column for cumulative upstream pipe length.

(a)
(b)
Figure 9. Attribute table results: (a) original tile layout; (b) new IDT routines layout in QGIS.
(a)(b)
Figure 10. Attribute tables for (a) original tile layout and (b) the recreated network.
Figure 11. Screen display result from running the “Tile Network Ordering” tool: (a) attribute table; (b) layout flow Lines.

Of the four new fields generated for each line segment from the Network Elevation Exports tool, the true length (“LENGTH”) and end point elevations (“Elev_first” and “Elev_last”) will be used as parameters for pipe sizing the drainage lines. In addition, the new field added (“FLOW_LENGTH”) from running the Network Flow Lengths tool highlights the cumulative flow lengths per connected link for each line segment in the network layout.

Figure 12. Screen display result from running the “Tile Network Ordering” tool: (a) attribute table; (b) tile layout flow ordering.
Figure 13. Attribute table results highlighting the four new fields from the “Network Elevation Exports” and a new field from the “Network Flow Lengths” tool, respectively.

Discussion

The five tools from the IDTs covered in this study present the second stage involved in a three-stage design process of drainage system design using the QGIS software platform. The first stage comprises a set of tools for facilitating the drainage layout, and the final stage comprises application tools for burying and sizing drainage networks.

After the digitization process of the drainage lines in QGIS (or any GIS), we recommend that intersection points in the layout network be checked for any corrections that can be done manually before using the first of these five tools: Hydraulic Network Fixer [E]. This check is necessary to minimize the errors of the digitization process, and if the settings for the default values as shown in figure 5 are to be used, the default values indicate by what length the digitized lines will be adjusted to compensate for discontinuities like dangles and overshoots in the drainage layout network. The unit is contingent on the unit of the data layer and its coordinate reference system. Moreover, this practice helps to prepare the digitized line vector shapefiles for use by the tools, regardless of their complexity.

By testing the network layout created using another software with the QGIS Topology Checker Plugin (fig. 9a), we further established the need to have all digitized lines checked for positional errors in QGIS, notwithstanding the novelty of the software used in creating the drainage lines. Even though the layout shows the presence of some topology errors, the commercial software likely has inbuilt proprietary algorithms that handle positional errors from digitization during the burying process.

The results from this study are limited to the outcomes from correcting the positional errors from digitizing lines. Since subsurface drainage systems are comprised of lines, the digitization of map-features such as points or polygons was not addressed. On a broader scale, the new tools can be utilized in contexts where the simulation of controlled and directed movement of substances or information is critical, like the effective and efficient management of wastewater and stormwater.

Conclusions

By improving on the capabilities of the QGIS Topology Checker plugin, which only identifies where topology errors exist in a vector layer and requires a tedious and time-consuming manual fix for all identified errors, the new set of tools proves to be a very easy and user-friendly approach to fixing positional errors resulting from line digitization in QGIS. A major advantage and novelty of these new tools is that they offer a complete combination of techniques and tools for identifying and fixing fractured line networks that enhance the visual flow simulation of fluids.

The case studies and results highlighted in figures 2, 7, 9, and 10 are indicative that the developed tools successfully generate topologically-sound, unidirectional networks that can be used to bury and size subsurface drainage systems. It also simplifies the painstaking task involved in the correction of digitized line vector shapefiles and reduces the level of GIS expertise required to produce networks for drainage system design.

The development of these tools underwent many changes and improvements based on feedback from test users. This development process will be continual. We appreciate all input and ideas to further improve the tools, and we likewise encourage the testing and use of these tools using data other than the ones provided.

Supplemental Material

Supplementary data for this communication can be found online at https://publish.illinois.edu/illinoisdrainageguide/illini-drainage-tools.

declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this study.

Acknowledgments

We would like to express our appreciation to several groups of undergraduate students in the ABE459-Drainage and Water Management course at the University of Illinois who have worked with the tools and provided insights that were used to improve them. This research was supported in part by the intramural research program of the U.S. Department of Agriculture, National Institute of Food and Agriculture, Hatch (accession number: 7005794). The findings and conclusions in this preliminary publication have not been formally disseminated by the U.S. Department of Agriculture and should not be construed to represent any agency determination or policy.

Nomenclature

IDTs = Illini Drainage Tools

QGIS = Quantum Geographical Information System

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