Contributor: Community Mapping Network, Marc Porter, Glen L. Porter and Sara Muir Owen
Location: Lower Mainland, British Columbia
Land Use: Settlement
Habitat Type: Watercourse/Riparian
Project Timeline:
Highlights
The Community Mapping Network, in partnership with the District of Mission, undertook a project to assess the accuracy of photogrammetry at delineating watercourses and their top of banks (ToB) in a selected test area: the Cedar Valley Comprehensive Development Plan (CVCDP) area of Mission, BC.
Determining the correct location of watercourses and their associated top of banks (ToB) is a necessary prerequisite for delineating the boundaries of Fisheries Sensitive Zones (FSZs). FSZs are comprised of both instream aquatic habitat and the adjacent riparian areas above the normal high water line of the stream or wetland. Defining FSZs is an essential planning component for establishing the minimum setbacks required for development adjacent to a stream.
Setbacks are areas of land and vegetation adjacent to watercourses that are to remain in an undisturbed state, throughout and after the development process (Chillibeck et al. 1992). The location and extent of these setbacks will be determined by the presence and proximity of a watercourse on or adjacent to a development site, by the presence or absence of fish in the watercourse and by the nature of the watercourse and surrounding vegetation. Provision of these setbacks may be a primary component of streamside protection regulations being developed under Section 12 of British Columbia’s Fish Protection Act (FPA).
Watercourse and ToB locations can be determined directly in the field using high precision GPS survey units or else interpreted remotely through photogrammetric analysis of high-resolution aerial photographs. The use of photogrammetry represents a potentially lower cost and time saving approach for delineating stream and riparian corridors over larger geographic areas. However, the accuracy of the existing photogrammetric methodology is presently untested and it is uncertain whether the technique has broad reliability.
Project Objectives
The Community Mapping Network, in partnership with the District of Mission, undertook a project to assess the accuracy of photogrammetry at delineating watercourses and ToB in a selected test area: the Cedar Valley Comprehensive Development Plan (CVCDP) area of Mission, BC. The primary objectives of the project were to- select a representative sub-drainage (project area) from within the CVCDP where the ToB boundary was delineated using photogrammetric techniques (Method One)
- ground truth the project area to locate the ToB and watercourses using Global Positioning System (GPS) technology (Method Two)
- compare the ToB boundaries and watercourse locations as determined by the two methods
- provide recommendations for delineating the boundaries of FSZs in British Columbia using the results of this assessment
- establish accurate “natural open space” boundaries for Cedar Valley so that the District of Mission can proceed with their Development Plan for the area.
Method 1
Black and white aerial photos of the project area were captured in two flight lines during leaf-free conditions using a 305-mm focal length camera. A digital 1:10,000 base map depicting hydrographic and ToB information within the entire CVCDP area was subsequently produced.Method 2
Ground-truthing of ToB was accomplished by placing numbered flags along the ToB boundary approximately 15 m to 20 m apart. The flag locations were then confirmed using GPS technology (i.e., Trimble Pathfinder Pro-XR with ± 5 m accuracy 95% of the time). Ground-truthing of stream centreline locations was established by walking all watercourses present within the project area.Eight study plots were selected for detailed comparisons of the two methods used for delineating watercourses and ToB within the project area. Within each site, major habitat and terrain features were described and any observed discrepancies between the two methods in determining streams and ToB were identified. Apparent reasons for these discrepancies were deduced from an integration of information obtained from the field (i.e., relative habitat and terrain conditions) and detailed examination of the aerial photo overlays. Differences between the two methods in depicting stream location and right and left ToB were determined for each site by measuring the horizontal distance between respective linework at 20 approximately equidistant intervals.
ToB and watercourse locations determined by photogrammetry (Method 1) differed considerably from ground-truthing with GPS (Method 2). Within study sites for which streams were correctly identified by both survey methods, photogrammetry misplaced stream locations by 7.6 to 11.6 m (on average) and maximum stream displacement relative to GPS location was as much as 25.5 m. Location of site ToB boundaries depicted by the two methods varied on average from 8.3 to 35.2 m, and maximum displacements were in the range of 18.4 to 70.2 m.
Serious photogrammetric errors in mapping for the study area included:
- failing to sufficiently extend ToB to the edges of low gradient floodplains along the creek corridor
- incorrectly mapping the actual locations of watercourses
- mapping watercourses that were not actually present
- missing several watercourses altogether.
A direct objective of the mapping work undertaken for this project was to establish “natural open space” boundaries for the CVCDP. These boundaries are intended to form the basis of a Memorandum of Understanding amongst the District of Mission, B.C. Ministries, and Fisheries and Oceans Canada that will chart the course for development in the area for at least 10 years and possibly longer. Previous efforts to map the CVCDP had generated public concerns that the indicated boundaries were not accurate and were, in some cases, inappropriate.
Now that the streams and area boundaries for the CVDCP have been mapped in an accurate and comprehensive manner using field based GPS, the District can proceed with the Memorandum, and the public can be assured that the job has been done using the best available technology.
Photogrammetry displayed considerable difficulties with correctly mapping streams and ToB boundaries within the study area across a range of different terrain conditions. However, the technique may be sufficient under certain limited conditions. These might include areas where a clear demarcation between a narrow forested ravine and urban landscape allows an easy assessment of stream location and associated ToB. Alternatively, open non-forested areas should also allow relatively easy interpretation of watercourse locations and ToB. Further work may suggest situations where photogrammetry could play an effective role in defining FSZs.
At present, however, there is no indication that photogrammetric interpretation displays the general requisite level of accuracy necessary for use as a detailed inventory tool. Streamside protection setbacks based solely on photogrammetry would likely be seriously erroneous, and require considerable readjustment by ground based measurement. Ground-based GPS surveying for stream locations and ToB represents a much more reliable method than photogrammetry. In addition, besides establishing stream locations and ToB boundaries, field assessments can provide information about important biological, chemical or physical features of a watershed.
Results from this project suggest that map interpretations based upon photogrammetry (even using very high resolution imagery) may fail to accurately capture and delineate small streams and wetlands, and may seriously misrepresent FSZs. However, high quality air photography (especially in the form of digitally rectified orthophotos) is increasingly becoming an accessible and attractive tool for urban planners. As such, landbase interpretations based on photogrammetry and other remote sensing techniques are likely to play a major role within regional planning exercises in the future. Low level aerial photography will provide significant detail over large areas, more quickly and perhaps more cheaply than intensive ground based surveys over the same area. As such it represents an important element in capturing some types of information required for sustainable community planning.
However, in terms of aquatic habitats the work here suggests that although high resolution photography represents an improved starting point for identifying potentially sensitive areas, it should not be used as the sole method to delineate stream centre lines, ToBs or FSZs.
Acknowledgements
Thanks to ShortReid Terrain Data Ltd. for develoment of the orthophotography for the area and Scott Resource Service Inc. for their interpretations of photographic and GPS data.
This case study was prepared by Marc Porter (Fisheries and Oceans Canada) for the ‘Planning Sustainable Communities Workshop’ sponsored by the Community Mapping Network (www.cmnbc.ca) and the Township of Langley. The workshop, held March 15, 2002, showcased examples of mapping as a tool for sustainable planning. These examples were compiled in the workshop’s summary document ‘Planning Sustainable Communities’ by Glen Porter of G.L. Porter Consulting with support from Brad Mason (Fisheries and Oceans Canada); Rob Knight (Ministry of Water, Land and Air Protection); Gretchen Harlow (Environment Canada); and, Kathleen Moore (Environment Canada). These case studies were later added to the BC Stewardship Centre Case Study Library by Sara Muir Owen with support from Rod Silver (Habitat Conservation Trust Fund).
References:
Chillibeck B., G. Chislett, and G. Norris. 1992. “Land development guidelines for the protection of aquatic habitat.” Department of Fisheries and Oceans Canada. Pacific Region. Vancouver, B.C., and B.C. Ministry of Environment, Lands and Parks, Integrated Management Branch, Victoria, BC
Contacts
Fisheries & Oceans Canada
http://www.pac.dfo-mpo.gc.ca/
Habitat Enhancement Branch
Brad Mason M.R.M.
Habitat Inventory Co
360 555 West Hastings Street
Vancouver, BC Canada
V6B 5G3
Phone: (604) 666-7015
Fax: (604) 666-0417
Email: masonb@dfo-mpo.gc.ca
Scott Resource Service Inc.
Jim Scott
31856 Silverdale Avenue
Mission, BC Canada
V2V 2K9
Email: scottres@telus.net
ShortReid Terrain Data Ltd.
Darren Heisler
Unit 1 – 20120 92A Avenue
Langley, BC Canada
V1M 3A4
Email: Jds@shortreid.com
Partners
For more information about community mapping projects, visit the Community Mapping Network at www.cmnbc.ca.