A 2.185Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE
Breaklines are vector features (lines and polygons) that are created to ensure the accuracy and cartographic quality of a topographic data product such as a digital terrain model (DTM) or orthophotography. Due to the site specific and/or regional variability of topography and manmade phenomenon associated with mapping projects, it is difficult to set universal standards for breakline development in order to achieve accuracy goals for contours or digital elevation models (DEMs).
Topographic datasets differ largely in the specification and accuracies to which they were collected and there is a wide range of complexities in each collection area. As a result, there is no "one size fits all" solution for every project when applying breaklines in order to improve LiDAR data.
For example, hydro flattening is the processing of a LiDAR derived surface (DEM or triangulated irregular network (TIN)) where mapped water bodies, rivers, reservoirs, and other cartographically polygonal water surfaces are flat; therefore, appropriately level from bank-to-bank on a sloping surface. Surfaces of rivers and long reservoirs demonstrate a gradient change in elevation along their length consistent with their natural behavior and the surrounding topography. The use of breaklines that ensure continuous downward sloping of water flow is termed hydro-enforcement.
In traditional photogrammetrically-compiled mapping, the hydro flattening process is accomplished automatically through the inclusion of measured breaklines in the DTM. LiDAR does not inherently produce breaklines, and LiDAR surveying returns scatter and is reflected over water causing unsightly and unnatural artifacts in the derived TIN and DEM. The process of hydro flattening typically involves the addition of breaklines along the banks of specified water bodies, rivers, ponds, and streams. These breaklines establish elevations for the water surfaces that are consistent with the surrounding topography and produce aesthetically acceptable water surfaces in the final derived DEM or TIN.
The use of breaklines ensures flattened bank-to-bank appearance of rivers and streams, as well as, a consistent downward slope referred to as "monotonicity" . Also, the use of accurate breaklines ensures the proper processing of contour lines that do not cut across water bodies and that closely delineate and portray stream and river slopes. The validation of breaklines derived from LiDAR demands a thorough inspection of the 3D vector data in conjunction with LiDAR and aerial imagery. To automate the inspection process, Michael Baker Jr., Inc. (Baker) has developed software called BKVAL to validate the breaklines.
Importance of Breaklines on H&H Studies
In most cases, supplemental breaklines are collected using a field survey or a photogrammetric process to enhance the realistic and aesthetic representation of terrain models or for enforcing proper contour delineation along the edges of natural and manmade features. For terrain data used in water related applications, proper breaklines and hydro-enforcement will often be crucial for correctly modeling probable flooding scenarios. Breaklines remove the improper accentuation of artifacts in the data that result from bare earth elevation interpolation effects in the DEM due to dense vegetation along the banks of streams and channels. When applicable the breaklines ensure accurate and continuous elevation at the crest of levees, and they improve the volumetric depiction of stream and channels by improving the definition of the beds and banks. Another advantage to the use of hydro-enforced terrains, that is often overlooked, is that it reduces post-processing of flood zone delineation, the cleanup of artifacts, and slivers in the terrain products, especially when flooding is contained in channels.
Validation of Breakline
The use of breaklines is a costly, delicate approach to "improve" terrain byproducts from LiDAR data. Breaklines inherently induce alterations in surveyed or directly collected data to make that data conform to a perceived reality. This conformance is more prominent for natural features than manmade ones such as transportation infrastructure. The purpose of breaklines is to ensure continuity of perceived features and to enforce the interpolation of the terrain to relative elevation values with minimal deviations from "true" measurements. Those minimal deviations turn into errors when their values exceed acceptable thresholds depending on terrain applications.
Logical constraints could also turn acceptable deviations into errors, such as breaklines for stream beds should not be higher than adjacent grade. BKVAL software facilitates the task of assessing those deviations and assists the analyst to focus their attention on areas of concern; an error of concern is associated to locations where exceptions to set parameters are justified due to the complexity of the natural landscape. However, more investigations may be needed to ensure breaklines are acceptable for use in developing terrain byproducts suitable for the intended applications.
Table 1: Vertical Consistency Report showing variance between adjacent vertices exceed set threshold
Visual inspection of the breaklines is a crucial step towards approving a quality, acceptable product. Breaklines should be visually compared to both a shaded relief produced from the acquired LiDAR and high resolution imagery that’s preferably acquired along with the LiDAR. The analyst checks the accuracy of the breakline positioning in comparison to the natural features, and the completeness of the types of breaklines specified by the client (e.g. culvert line, top of levees, single-line drains for small rivers and double-line drains for large rivers, etc.)
About the software Application
BKVAL software checks for the following:Monotonicity
Hydro breaklines should flow and should not undulate.
Vertical Consistency
Breaklines should be 3D polylines; vertices should have a Z value.
Vertices should not have a 0 elevation where not applicable.
Vertices should not have excessive min. or max. z-values when compared to adjacent vertices. Seldom occurrences are validated against imagery (e.g.: drops, weirs).
Vertical variance between breakline vertices and LiDAR DTM should not exceed a threshold value determined in the project. Occurrences are validated against imagery for exceptions, i.e. dense riparian vegetation along banks.
Topology
Breaklines should not intersect unless the same elevation; but culvert bridges and overpasses breaklines may intersect hydrographic features.
The inputs for the software application are
LAS Files covering the project area
Breakline Shape File
Vertex Tolerance
LiDAR Tolerance
Output File Location
The software application will convert the breaklines vertices to points and compare each point to each other point within and across breaklines and surrounding ground values. The software application also checks monotonicity by comparing the trend of first upstream vertex being higher than the last downstream vertex. The elevations for each vertex decrease steadily as the stream flows downstream.
The program generates two (2) Excel reports showing the results for validation of monotonicity and vertical consistency. A sample of the two (2) reports is shown in Table 1 and Table 2. Figure 1 shows that the breaklines are not hydro-enforced–they are not sloping in the downstream direction. The red points indicate the sinks along the breakline. In Figure 2 breakline vertex elevations do not always match within the tolerance of each side of the stream and also have excessive minimum and maximum elevations.
The software analysis is the first step in the breakline review process; the results generated help the analyst detect overall systematic quality issues in the development of the breaklines and/or focuses the attention to areas of concerns that may or may not be deemed as justified exceptions after further investigations.
Summary
Breakline production is a time consuming and costly task that is not yet fully automated. The quality and completeness of breaklines varies depending on complexity of the terrain landscape and the technical skills and experience of service providers. Using subpar breaklines deteriorates the quality of terrain models and may render them unusable for analysis and scientific applications.
To ensure adequate and sound breaklines, Baker developed BKVAL, a software application that semi-automates the QA/QC review process. BKVAL saves time and improves the objectivity of the reviews by detecting systematic errors and/or by focusing the attention on areas that require manual/visual inspections. BKVAL allows flexibility in setting the tolerance of the review parameters due to variation in requirements in the scope of work and the end use of the terrain data. Additional improvement to the flexibility and parameter setting of the program are planned in future releases. Baker’s focus is to improve our clients’ return on investment by validating and improving the quality of breaklines needed for various uses.
References
FEMA’s Memorandum for Regional Risk Analysis Branch Chiefs, Procedure Memorandum No. 61: Standards for LiDAR and Other High Quality Digital Topography, Effective Date September 27, 2010.
USGS–LiDAR Base Specifications Version 1.0–Techniques and Methods 11-B4.
American Society for Photogrammetry and Remote Sensing (ASPRS), ASPRS Guidelines, Vertical Accuracy Reporting for LiDAR Data, version. 1.0, May 24, 2004.
Dr. Srinivasan "Srini" Dharmapuri has over 26 years of extensive, wide-ranging experience within the Geospatial industry; most notably with LiDAR, Photogrammetry, and GIS. Currently, he is working as a LiDAR Scientist with Michael Baker Jr., Inc., and he is also the chair of the Mobile Mapping Committee ASPRS.
Pascal Akl, GISP is a skilled GIS data analyst, geodatabase developer, topographic data SME and RS imagery interpreter. For the last 7 years with Michael Baker Jr., Inc., Mr. Akl has worked extensively on providing GIS analyses and technical support.
A 2.185Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE