The orthoimage data set is created with the most accurate control data available at the time of creation. The imagery has been corrected with either provincial and aerial triangulation data or, if not available, the most accurate NTDB data.
The objective is to obtain accuracy of 30 metres or better in the South and 50 metres or better in the North for a 90% level of confidence. The accuracy is evaluated for each orthoimage data set. The control data, which has been extracted from sources as mentioned above, can consist of road intersection vector data or lake/island centroids. In some cases the intersection of the centre line of surficial river may have been used. The control points are distributed homogeneously within the image in specific sectors. Sectors are located in the surround of the image, in the lowest and highest elevation area of the image, and in all other areas in which overlapping image base sectors are located.
The objective of the national orthoimage project is to produce a complete set of cloud-free (less than 10%) orthoimages covering the Canadian landmass using Enhanced Thematic Mapper (ETM+) data from the Landsat-7 satellite. Production, which began in fall 2000, will span a period of three to five years. The purpose is to promote the use of geomatics and educate by providing a Web and file transfer protocol (ftp) site that distributes Canada's geospatial data.
Landsat-7 data are collected from a nominal altitude of 705 kilometres in a near-polar, near-circular, Sun-synchronous orbit at an inclination of 98.2 degrees, imaging the same 183-km swath of the Earth's surface every 16 days. Each frame is denoted by a sequential Path and Row determined by the 16-day repeat cycle. Landsat sensors require 233 orbits. The Rows, indexed east/west, are generated by partitioning each Path into 23.92 seconds of spacecraft time in both directions at the equator, resulting in 248 Rows per complete orbit. The Landsat sensors continuously collect data which are segmented post-collection on the ground, using telemetry ephemeris data, into individual framed scenes. The orbital tracks can drift over time due to a variety of factors and are adjusted when necessary. The framing is unique for each orbit. Accordingly, the frame locations are not exact, but are within a tolerance of movement off the original satellite orbit. The satellite orbit results in a coverage side overlap of a minimum of 7.3% at the equator increasing to approximately 85% at 80 degree of latitude.