Data Specification on Orthoimagery – Draft ... - INSPIRE - Europa

INSPIRE Reference: D2.8.II.3_v2.0
TWG-OI <strong>Data</strong> <strong>Specification</strong> on Orthoimagery 2011-06-15 Page 60
be necessary to perform on-the-fly re-projections to transform from the source coordinate reference
system (in which data is stored) to the one chosen for display. This implies unavoidable degradation of
source data on the screen, as any additional geometric transformation involves resampling processes.
It is worthy to mention here that the most direct way to provide raster data sets through a download
service according the previous recommendation would be to store them at data sources using
geodetic coordinates based on the Grid_ETRS89-GRS80. This would make possible downloading the
original harmonised data free of any additional transformation.
In the future, production of Orthoimagery data sets should be based on the common Grid_ETRS89-
GRS80 instead of national grids, since the unavoidable transformation between the grids has an
inherent loss of quality.
The planar representation of geodetic coordinates introduces unusual distortions as the latitude
parameter increases, as a result of the convergence of meridians. This may cause undesirable effects
in areas located at high latitudes, which become especially important in areas near the Polar Regions.
Particularly, the longitude axe is rescaled causing a stretching of the grid in the WE direction when it is
represented in geodetic coordinates (visually this is perceived as if the grid is flattened in the NS
direction).
As a consequence of this convergence, the volume of data needed to store raster information in
geodetic coordinates increases as the latitude does, since the number of grid cells per area unit
becomes greater.
EXAMPLE While 1 arcsecond in longitude corresponds approximately to 31 meters at the Equator,
it corresponds to 25 meters in Gibraltar (southern part of Spain - latitude 35 degrees)
and only to 10 meters in North Cape (North Scandinavia - latitude 71 degrees).
Therefore, for the visualization of such data sets it is recommended the use of any map projection
which is suitable to the specific geographical area to be rendered as well as it is allowed by the
Regulation (EU) No 1089/2010, of 23 November 2010, on interoperability of spatial data sets and
services.
Consequently, on-the-fly projection transformations are needed for view services serving these data
sets.
Recommendation 16 Real time re-projection is recommended for the view services serving
INSPIRE raster Orthoimagery data sets aimed at serving global purposes.