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Usability of the 'Zoned Geographic Grid' (Grid_ETRS89-GRS80)

Use the present discussion topic to clarify aspects related to the use and application of the INSPIRE 'Zoned Geographic Grid' (Grid_ETRS89-GRS80).

Definition and properties

The 'Zoned Geographic Grid' is defined in Section 2.2.2 of Annex II of the Implementing Rule as regards Interoperability of Spatial Data Sets and Services (IR ISDSS).

It is a multi-resolution geographic grid based on the ETRS89-GRS80 geodetic coordinate reference system. It follows a structure analogue to DTED (Digital Terrain Elevation Data), dividing the world into different zones in latitude, to mitigate the effect of convergence of meridians.

It was proposed during the development of the INSPIRE Annex II & III data specifications.

Use of the grid

Either the 'Equal Area Grid' or the 'Zoned Geographic Grid' (defined in Section 2.2.2 of Annex II of the mentioned Implementing Rule) shall be used as a geo-referencing framework to make gridded data available in INSPIRE (data provision for INSPIRE), unless:

a) Other theme-specific grid is specified for a data theme;

or

b) For grid referencing in regions outside of continental Europe, where Member States may define their own grid under certain conditions.

In particular, the 'Zoned Geographic Grid' may be used as a geo-referencing framework when gridded data (raster data) is delivered using geodetic coordinates (see section 1.3 of Annex II of the mentioned Implementing Rule), mainly suited for reference data (such as elevation or orthoimagery).

 

NOTE: Please, look at the following discussion topic in the parent cluster group - where the topic was introduced.

https://themes.jrc.ec.europa.eu/discussion/view/8899/copernicus-rda-eu-dem-egcs-naming

  • Peter STROBL

    Thanks Jordi, for splitting the discussion into more digestible piecessmiley!

    What regards this group here: I'm looking for implementations of this grid to serve as a role model for future data sets that e.g. COPERNICUS might come up with.

    - Does anyone out there have a pan-European data set that is compliant to the INSPIRE Grid_ETRS89-GRS80 specification and that can be shared? Which SW do you use to process it?

  • Jordi ESCRIU

    Probably I am not wrong in saying that no Member State (MS) has implemented / distributed a dataset using the 'Zoned Geographic Grid' yet.

    As already said, it was a proposal developed in collaboration between TWG-EL and TWG-OI during the drafting process of INSPIRE Annex II & III data specifications.

    The 'Zoned Geographic Grid' is defined in geodetic coordinates - avoiding projections - and follows a structure analogue to DTED (Digital Terrain Elevation Data). It is a hierarchical grid and takes into account most typical resolution levels of EL and OI datasets.

    The aim of this proposal is to establish a common grid at European level in order to promote interoperability and prevent future cross-border issues (e.g. common grid referencing framework to avoid possible misalignments, inexistence of different projection zones, proper resolution levels). 

    Note that this grid may be used for the provision of raster (gridded) reference data to INSPIRE, i.e. this is optional and does not force MSs to transform the dataset to this grid.

    More about Use of the grid...

    As mentioned in the introduction to the present discussion topic, the IR ISDSS states that either the 'Equal Area Grid' or the 'Zoned Geographic Grid' shall be used for making gridded data available for INSPIRE within the scope of continental Europe…

    Unless, otherwise specified at theme-specific level! - Making use of such an exception:

    In the case of the Elevation (EL) theme (Section 1.7.2 of Annex III of the IR ISDSS):

    (1) By way of derogation from the requirement in Section 2.2 of Annex II, any grid compatible with one of the following coordinate reference systems may be used for making gridded Elevation data available:

    • two-dimensional geodetic coordinates (latitude and longitude) based on a datum specified in 1.2 of Annex II and using the parameters of the GRS80 ellipsoid;
    • plane coordinates using the ETRS89 Lambert Conformal Conic coordinate reference system;
    • plane coordinates using the ETRS89 Transverse Mercator coordinate reference system.

    The grid specified in Section 2.2.1 of Annex II shall not be used.

    A similar derogation is also included for the Orthoimagery (OI) theme (Section 3.5.2 of Annex III of the IR ISDSS).

    Summing up...

    MS are able to choose between these two grids ('Equal Area Grid' or the 'Zoned Geographic Grid') which to be used for the provision of gridded data.

    But, at least in case of the EL and the OI themes, they are also allowed to provide the information using their National Grid System, if this system is based on one of the 2D CRS allowed by INSPIRE for data provision (those specified in Section 1.3.2 of Annex II of the IR ISDSS), except ETRS89 Lambert Azimuthal Equal Area – e.g. the 'Equal Area Grid' shall not be used.

    Going back to the origins of the 'Zoned Geographic Grid':

    • It is clear that transforming existing data to this new grid requires interpolating original data with the subsequent degradation.
    • But it is a proposal for future convergence at European level to achieve interoperability.
    • The underlying idea is to promote that new raster reference datasets are produced based on this grid. In this case, data would be originally stored using this common framework from the beginning. Hence, no more reprojection, interpolation and degradation will be necessary for INSPIRE data provision.
    • MS are not currently force to use it for (gridded) data provision. Using it is ‘almost optional’.

    The adoption of this proposal shall be appropriately tested. Obviously It has some advantages and drawbacks...

    And the INSPIRE Thematic Clusters initiative is a good place to discuss it - Your views are welcome!

  • Jordi ESCRIU

    I copy here the input from Guillermo Villa in the discussion topic about misalignments between orthoimages generated by diferent data providers (Orthoimagery subgroup).

    Original text from Guillermo:

    In orthoimagery, we should differentiate two different cases:

    Case 1: Aerial orthophotos. The main problems here are: data production, storage, compression, efficient multiscale serving through Internet (WMS and WMTS services) and quick and flexible visualization and layer overlay with simple web viewers or GIS clients. WMTS servers introduce the need for pyramidal tiling and to work in a single projection. Consequences: It is not practical to have data in different UTM zones, WMTS client and server must work in the same projection, etc…

    Case 2: Satellite images. Remote sensing tendencies in the last years are: multitemporal  and multiscale analysis, integration of data from different sensors and massive parallel and cloud processing of great amounts of data (Big Data technologies).

    In both cases, 1 and 2,  the non-alignment of the pixel borders of georreferenced imagery causes very big interoperability problems:

    - For orthophotos: the need to apply multiple reprojection and resampling operations (causing great costs in processing time, storage space and also degradation of image quality).

    - For satellite imagery: impossibility to directly compare radiometric values for different dates (with very negative consequences in multitemporal analisys, change detection, etc.)

    These interoperability problems happen not only with datasets produced by different producers, but also with single producers. For example, Landsat 8 images are being geometrically corrected in different UTM zones (and ESA plans to do the same for Sentinel 2 images).

    These problems are seriously hampering the development of new technologies, operational applications and business models.

    For orhophotos and satellite images, the solution could be the same: we should reach an agreement on a single grid to produce, store, process, analyze, compare and serve orthoimagery. This grid should be fixed, multiscale (pyramidal), “nested” (2 by 2 pixels of one level of the pyramid should be exactly contained in one pixel of the upper level), assure the alignment of pixel borders at all pyramid levels, and cover the whole earth (or at least the biggest part of the inhabited areas) in a coherent and efficient way. The grid should use a conformal projection in order to avoid “strange” looking of common features like buildings, roundabouts, etc. and a disagreeable kind of “perspective effect” in areas other the equator.

    Inspire recommends the use of a common grid to address some of these problems, but the recommended Zoned Geographic grid does not comply with some of the requirements mentioned above, and does not contain a tiling schema.

    In the last times a “de facto” standard grid has emerged in WMTS services: the one of the tiling schema based on the “Spherical Mercator” conformal projection (EPSG:3857) used by Google Maps, Bing Maps, Yahoo Maps, Open Street Maps and other tiled data and API providers. This grid and tiling schema is fully documented and supported by a great number of software open libraries.

    WMTS is in the process of becoming an official OGC standard (“WMTS simple implementation”), and fixes the projection and TileMatrixSet definition.

    The general acceptance of this tiling schema opens the way for a solution of all the problems cited before. Nevertheless, two remaining problems should be addressed:

    1) WMTS 256x256 pixels tiles are way too small to be practical as production, storage, and processing units. One easy solution is to use tile footprints of one level and pixel sizes of a different level. E.g: use Level 10 tile footprint, but Level 14 9.5546m GSD, so this “supertiles” are 4096x4096 pixels.

    2) “Supertiles” should have overlaps between them, to avoid (or at least minimize) border effects when we perform operations like filtering, classifications, segmentations, etc. These overlaps should have a width of 2n pixels (256, 512, 1024,…) in order to maintain nested pyramidal overlaps. Overlap pixels should be added at the right and below the supertile, in order to maintain the upper left corner position and avoid georreferencing burden.

    A solution in this direction could be proposed to Inspire and should be proposed also to Copernicus and ESA.

    According to him, the existence of this misalignments causes real interoperability problems and the only way to avoid them is to establish and use a common grid, without using any projection with zone divisions and preferably conformant.

    He also states that the 'Zoned Geographic Grid' (the one proposed as common grid, at least for EL and OI) should be based on the simple tile schema defined by the WMTS "de facto" standard.

     

    In my opinion it is clear that a common grid shall be defined for such reference data coverages (especially for EL and OI data). And the proposal of the 'Zoned Geographic Grid' in INSPIRE should be adapted to best fit the user and application requirements.

    In principle, this would be possible if Member States has not started yet implementing this common grid in geodetic coordinates.

    Please, provide your opinion!

  • Peter STROBL

    Thanks Guillermo for your contribution!

    I am very grateful that you stress the need of common grids not only for data exchange but also as a common work basis. This is an aspect which I found largely missing in most discussions. I also agree (from my own experience) that UTM will not be a viable projection to solve the issue for continental scale work and that a grid convention must be based on

    1. a planar, orthogonal coordinate system,
    2. be regular and
    3. come with a limited, hierarchically well structured set of resolutions.

    Now the 'Zoned Geographic Grid' probably shouldn't be measured against these criteria, as it was meant for something else (i.e. data exchange - not processing).

    For Europe I have been working for more than 10 years very successfully with the system proposed by Wirthmann et al. in "Reference Grids for Europe" (pp.39-46). It foresees a quad-tree structure with all powers of ten, their halfs and quarters from 1m-1000km in 19 levels. For me it's the best possible solution I've came across.

    My idea would be build a global system of reference rasters by combining a set of zoned (but continental) projections such as recently proposed by Marschallinger et al. (2014) with a structured hierarchical system of grid levels such as the above mentioned quad-tree approach.

    If there's anyone out there interested in developing this idea further I'm happy to join force!

  • Guillermo VILLA

    By Guillermo VILLA

    The point is that "WMTS simple" has become a de facto standard. This means that every WMTS service is obliged to cache tiles in this system if it wants to give a good service to clients that ask for these kind of tiles (most of them). So using a different projection and tiling schema would imply duplicating efforts. As they say: "If you can't beat them, join them"...

  • Jordi ESCRIU

    Engage people participating in the OGC WMTS standards seems like a good idea.

    I will try to contact Joan Masó.

  • Guillermo VILLA

    By Guillermo VILLA

    That would be great. I think we should begin to unify the solutions (projection, tiling,...) for processing, storage and services if we want to avoid duplicating (or multiplying) efforts, an arrive to cost effective processes.

  • Jordi ESCRIU

    I have recently published this piece of news related to the present discussion topic:

    The Web Map Tile Service (WMTS) Simple Profile [13-082r1] has been recently approved by OGC Technical Committee.

    https://themes.jrc.ec.europa.eu/news/view/23130/web-map-tile-service-wmts-simple-profile-13-082r1-approved-by-ogc-tc

  • Peter STROBL

    Could you provide a quick overview of what "WMTS simple" contains in terms of allowed projection(s), raster and tile sizes? Is there a good explanation somewhere?

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Elevation, Ortho & Grids

Elevation, Ortho & Grids

INSPIRE Thematic Cluster Elevation, Orthoimagery, Reference systems, Geographical grids - Join this group to share your knowkledge, learn and collaborate in solving issues related to the Elevation, Orthoimagery, Reference systems and Geographical grids themes