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Forest Health Information Management

Standards for Detailed Aerial Survey Accuracy and Digitization

April  2003

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Introduction

Helicopter/GPS (heli-GPS) fixed surveys are the operational benchmark for accuracy, delivery time and cost for detailed aerial surveys at this time.

This standard provides GPS co-ordinates for detected centres of red attack with a horizontal accuracy of approximately 20m. Errors of commission (i.e. identifying a centre of red attack where there is none) are extremely low (<5%); errors of omission (identifying a non-infested site when it is infested) have not been measured, but are dependent on the effort expended to survey a particular area. If complete coverage is achieved, this error rate is probably less than 5% (i.e. more than 95% of the infested sites are located). In terms of image resolution, the level of accuracy consistent with the helicopter/GPS surveys is obtained with images produced at 1m pixel (50-cm being ideal) resolution.

In general, the aerial detection method must be able to identify both the number and location of individual red tree crowns and the species of tree killed.

The final product of this mapping exercise shall be a map (and GIS files) showing:

·      The location(s) of red attacked trees, pest codes, and tree counts (if spots - +/- 10%) or severity (if polygons +/- 5%) with GPS coordinates adequate to allow ground crews to find the centre of an infestation site with +/- 25 m accuracy

·      Infestations will be depicted as either points (spots) with tree counts or polygons with infestation severity (in 10% increments) (see digital data standards below)

·      The perimeter of area covered by the survey

The estimated unit cost for the standard detailed survey (helicopter/GPS) is $0.15/ha (product = map delivery).

GPS Standards

Aerial Surveys within +/- 20m horizontal accuracy. Based on the information in the document BC Standards and Specifications and Guidelines for Resource Surveys Using Global Positioning System (GPS) Technology Release 3.0 - Section E - Autonomous GPS Guidelines (BMGS GPS Resource Specifications Release 3.0), positional accuracy for bark beetle detection usually falls under the category of "non-critical" since the allowable horizontal errors for collecting point data can be greater than 10m. Thus, at minimum, locations recorded by hand-held autonomous GPS receivers (i.e., recreational use receivers) provide an acceptable level of accuracy when under clear tracking conditions. As noted in the guidelines above, some caution in their use is required to minimize error. In most cases, the objective of the survey is to obtain the location of the "infestation centre" within 25 m rather than obtain the precise location of each infested tree within the infestation centre. For improved accuracy, surveys should use differential GPS or high-end autonomous GPS receivers whenever possible.

Spatial Data Standards for Detailed Aerial Surveys

Forest Practices Branch has modified the Aerial Overview Digital Data standards to accommodate more detailed data obtained from operational aerial surveys in order to archive the detailed information in a standard format. Detailed aerial surveys and other surveys can be accommodated by the Forest Health Data Model currently accepted as the data standard for input of forest health data into the Integrated Land Management Bureau's Land and Resource Data Warehouse (LRDW).

Existing Aerial Overview Digital Data Standard:

Detailed Aerial Overview Digital Data Standard: As above with the modification of the SEVERITY attribute to designate the more precise estimate of percent infested within polygons sketch mapped, extracted from remote sensing or recorded on a GPS.  The Severity Codes are one-letter and are stratified by increments of 10%:

Metadata Requirements are the same as the Aerial Overview Digital data, namely:

Minimum Metadata Specifications for All Provincial Aerial Survey Data

All IGDS and Arc/Info data will be in the following format:

Projection: Albers Equal Area
Datum: NAD 83
Units: Metres
Spheroid: GRS 1980
1st standard parallel: 50 0 0.000
2nd standard parallel: 58 30 0.000
Longitude of central meridian: -126 0 0.00
Latitude of true scale: 45 0 0.000
False easting (metres): 1000000.00000
False northing (metres): 0.00000

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Last updated on April 19, 2007
The contact for this web page is: tim.ebata@gov.bc.ca