Tile Server

Coordinate Reference Systems and Projections

The data stored in the database and in raster form is very precisely referenced using Canada's official coordinate reference system, NAD83(CSRS) and vertical datum, CGVD2013. Some elevations are measured with respect to the local chart datum, and are calculated using a shift grid from CGVD2013 heights.

Due to isostatic rebound and plate tectonics, all positions move with respect to the reference frame (ITRF/WGS84), so they must be accompanied by an epoch, or date, with describes the date at which they are comparable. Positions collected in June 2023, for example, are given the epoch 2023.5, and transformed to 2010.0, which is the standard epoch used by a variety of organisations with in the federal public service (the province of British Columbia uses 1997.0 for Vancouver Island and 2002.0 for the mainland).

The Mapnik configuration file handles these transformations for rasters using proj, the official shift grid provided by NRCAN and a pair of epochs. The following proj configuration is for a map of a Calvert Island beach located in UTM zone 9, produced in June, 2023.

   +proj=utm +zone=9 +ellps=GRS80 +units=m +no_defs +type=crs +grids=ca_nrc_NAD83v70VG.tif +t_final=2010.0 +t_epoch=2023.5

• The +proj and zone parameters indicate that the map is in the UTM projection, zone 9.
• The ellps parameter gives the ellipsoid as GRS80, which is commonly used with WGS84, the coordinate reference frame used by the GPS system.
• The grid parameter provides the name of a grid shift file, wherein each pixel represents the distance and direction a coordinate should be displaced per unit time.
• The t_epoch parameter provides the date of origin of the data, in this case, the date it was produced.
• The t_final parameter provides the target epoch to which the data will be transformed using the grid shift file, in this case the standard epoch, 2010.0.

The display map is declared as the standard "Web Mercator" projection (EPSG:3857, formerly EPGS:900913):

   +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +wktext +no_defs +over

The tile generator will use this pair of configurations, transforming the map from one to the other so that it can be displayed with a high degree of precision.