i) Geographics (Latitude and Longitude)
ii) Universal Transverse Mercator (UTM)
iii) BC Albers Equal Area, with parameters of:
Central meridian: 126:00:00 West longitude |
First Standard Parallel 50:00:00 North latitude |
Second Standard Parallel 58:30:00 North latitude |
Latitude of projection origin 45:00:00 North latitude |
False northing 0.0 |
False easting 1000000.0 (one million meters) |
Except for Geographics, all map projections (including both UTM and BC Albers) introduce distortion when representing features from the curved surface of the earth onto a flat map. This distortion introduces errors when analysis is done on the map features.
BC Albers preserves area, but distorts shape and distance. UTM preserves shape and direction, but distorts area and distance. Appendix A quantifies some of the distortions associated with both UTM and Albers as compared to Geographics. Selection of a map projection for use should take both the type and degree of distortion into consideration.
Although UTM has better positional accuracy and is the most commonly accepted mapping standard world-wide, it divides the earth into six-degree zones of longitude. BC includes five of these zones, or separated projections. Handling the separated projections using current GIS tools for analysis of areas which span the zone boundaries presents technical difficulties and has prompted the need for a seamless map projection for the Province; BC Albers. Note that extending the UTM projection zones beyond their defined six-degrees of latitude will introduce errors greater than those stated in Appendix A..
Specific Standards and Guidelines
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Below are specific standards for using map projections in various situations. The situations include the use of map projections for local or operational storage in support of mapping operations, for archive of published maps as a source for data exchange (commonly in a warehouse), for original data capture, for analysis of map data, and for presentation of map information. In the case of analysis, interim guidelines are provided because most current analysis tools (i.e. GIS's) are not able to work effectively in the standard, namely Geographics.
Standard for Local Working or Operational Storage (Internal Use)
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For mapping operations the map projection is defined by the data custodian based on local (i.e. ministry) needs. A move to Geographics as analytical software becomes available is recommended. The standard does not address concerns that might relate to having more than one standard map projection and its impact on mapping operations. As example, moving from one projection to another involves a transformation. Transformation tools can introduce systematic errors when moving from one projection to another, that is, effectively changing the data. In an operational mapping environment involving updates to existing maps this could cause problems.
Standard for Archive/Exchange (Published/Warehouse)
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The Corporate standard is Geographics for Exchange. It is recommended the local projection also be made available as a client option. The latter should be accompanied by "user beware" statements as necessary (e.g. transforming distances associated with dynamic segmentation in BC Albers will lead to errors in another projection; or areas associated with UTM are in error).
Standard for Original Data Capture
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For all original data capture, involving line or polygon features being georeferenced to either the TRIM or Provincial 1:250,000 topographic base maps, compile using UTM to maintain consistency with Provincial topographic base mapping standards, thus ensuring data can be overlaid.
Standard for Analysis
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The Corporate standard for analysis is Geographics. It is recognized that most current tools are unable to support analysis in Geographics. Thus an interim set of guidelines for use of map projections is required.
Interim Guidelines for Analysis
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It is generally accepted that analysis for planning purposes, involving large areas, and where the key data involved is in a small scale (1:50:000 and greater), errors introduced by using either the UTM or BC Albers map projections is not of consequence. The guideline for practical purposes is to use the projection most convenient for your immediate analysis.
For larger scales, or operational analysis where accuracy, as influenced by map projection (see Appendix A), is a concern the following additional guidelines are provided:
i) When concerned with shape of features, spatial accuracy of features within an area (including direction or distance), or with analyses involving geometric constructs (e.g. buffers defined around points, lines or areas; analysis involving point or area grids, use of bounding boxes) and the required accuracy is supported by the data, use UTM.
ii) When concerned with the accuracy of area summaries of mapped polygon features and the required accuracy's are supported by the data, use BC Albers.
Summary of Interim Map Projection Guidelines for Analysis1
Table 1 - Summary of Interim Map Projection Guidelines for Analysis1
|
Scale \ Use |
Area Summaries of mapped polygons |
Area Summaries of mapped polygons involving overlays |
Area Analysis using geometric constructs (with or without overlays) |
Intersection Analysis involving overlays |
Linear Analysis |
Coordinate Geometry |
|
Key Sources are small scale (>=1:50,000) / focus is regional planning |
BC Albers (UTM) |
BC Albers (UTM) |
UTM (BC Albers) |
UTM (BC Albers) |
UTM |
UTM |
|
Key Sources are large scale (<1:50,000) / focus is operational planning |
BC Albers2 |
BC Albers2 |
UTM |
UTM |
UTM |
UTM |
1The less preferable projection is shown in brackets.
2Where area measurements are better than .08ha in 100ha
Standard for Presentation
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The map projection is to be defined by analyst based on presentation objectives.
Copyright © 1997 Province of British Columbia
Published by the Resources Inventory Committee