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Usefulness of Contour Lines transformed to EVRS from a national height reference system

One of the core spatial objects for the theme Elevation in case of vector data is the Contour Line. The description of the morphology of surfaces, both for terrestial elevation & bathymetry, is based mainly in this spatial object. Each contour line describes the points in the surface which has the same elevation value.

For practical purposes, the value selected to draw the contour is always a whole number, commonly referenced to the official national height system of the Member State. However, since INSPIRE demands the use of the European Vertical Reference System (EVRS), there is a need to transform existing data to such common reference system.

Going back to the contour line, the original elevation whole number in the national system shall be transformed to EVRS, so that the resulting elevation value is not necessarily an exact number (whole number).

EVRS contour lines obtained through this process may be used to perform calculations and analisys, but it would be problably unsuitable or strange to use them as background information for supporting visualizations.

Do you think that usefulness of Elevation vector data for INSPIRE purposes is compromised?

A solution for this would be to recommend the provision of EVRS contour lines calculated / generated from a  DTM in EVRS (grided data), assuming the corresponding loss of quality.

At the moment:

  • Elevation data shall be at least provided as gridded (raster) data for describing land elevations (i.e. provision of vector data may be considered as optional in this geographical scope, if gridded data is provided instead).
  • In case of bathymetry, Elevation data shall be provided either as gridded or vector data.

 

  • JAVIER GONZALEZ MATESANZ

    By JAVIER GONZALEZ MATESANZ

    Totally agree Jordi. Anyway, a clear rules to transform national data to EVRF2000 should be written somewhere.

    Contour regeneration would not be a problem as far as can be obtained from DTM but the collateral efects are still not known. In the Spanish case, an offset of 49cm must be applied to have EVRS ellevations.

    Then, yes. The richness of true 3D vector data of national agencies is lost in order to be consistent with elevation models and that models could have two different transformations: a) the rigorous tilted plane as published in crs.eu or b) a simple elevation offset in case the differences along the country were neglectables

  • Jordi ESCRIU

    Javier,

    Thank you very much for your input and opinion.

    Any other views on this topic?

    Jordi

  • Marc ROESBEKE

    Jordi,

    Yes, I agree that bathymetric elevation data shall be provided either as gridded or vector data.

    Tidal waters will use LAT as height reference. This is in line with the recommendation from IHO. IHO has not defined the procedure of how to realize LAT. The member states are responsible to define LAT. It is also upto the member states where the definition of tidal waters ends and where non-tidal waters begins.

    EVRS does not surround a non-tidal sea area. Closest is the link at coastal tidal stations where MSL, LAT and EVRS values are available. If MSL-EVRS relation is available, then the depths can be reported in EVRS and in metadata the relation between MSL and EVRS should be given. Displaying in EVRS allows a smooth transition to adjacent rivers.

    Displaying depth values in rivers can be done using EVRS. The offset between a common riverwaterlevel in EVRS height can be used as reference. Using the same philosophy of LAT, this common level should have a such a value that only 5% of all times the actual water level is less. (setting this standard is for example a task of the North Sea Tidal Working Group).

    If someone wants to use land reference (such as EVRS) to describe sea-floor data, it is possible to consider these data as "height" with negative values. It means that, as far as safety of navigation is not concerned, the bottom of the sea can be described exactly like any topographic surface on land.

    Marc

  • Jordi ESCRIU

    Thanks Marc,

    This is a very clear explanation about the possible scenarios for bathymetry data, either regarding the sea-floor (tidal areas / non-tidal areas) or the bed of navigable rivers. And good to know that the Elevation Technical Guidelines are in line with it!

    As you said, in bathymetry - except in few situations where EVRS may be used to express such surfaces in the same way and as a continuation of the land-elevation surface - other vertical reference systems and references shall be used.

    My first post was more focused on the fact that existing contour lines (vector data) in national systems have to be transformed to the INSPIRE vertical reference systems, and using the resulting transformed data may be accompanied of some problems (some examples referred above).

    Perhaps its easy to expect such problems in the land-elevation scope, where EVRS shall be used. And they may compromise the benefit of providing such vector data from Member States.

    However, I can also think of e.g. two vector bathymetry data sets from different Member States (MS), both describing the see-fllor in a tidal area. Each using their corresponding LAT, defined by the MS. 

    Should not we expect interoperability problems there? (i.e. transition from LAT 1 to LAT 2 probably is not a smooth one) - But may be this would be a different issue than the former one, and also affecting to covarage (raster) data as well...

    Jordi

  • Marc ROESBEKE

    Jordi,

    Yes, there are some interoperability problems with vector bathymetry data sets from different Member States. Inconsistencies in the encoding practices for ENCs (Electronic Navigational Charts) exist between Hydrographic Offices.

    In Edinburgh, Scotland at the INSPIRE Conference 2011 from 27th June to 1st July 2011 I gave a presentation about harmonisation of the ENC cells across the boundaries of neighboring countries. In that presentation you will see the testing results of the tool "ENC Harmonisation Checker". (http://inspire.ec.europa.eu/events/conferences/inspire_2011/presentations/workshops/272/1600%20Tues%20kilsyth%20Roesbeke.pdf)

    This tool can be used to compare adjacent ENC cells to determine whether there are inconsistencies between them. The tool can be used by the hydrographic offices of neighboring countries in order to address how they might harmonise ENC cells across their boundaries.

    Marc

     

     

     

  • Marc ROESBEKE

    Jordi,

    I send you the PPT file, because in the PDF file there are a few missing slides.

    Marc

  • Marc ROESBEKE

    Jordi,

    You'll find the PPT file in the Category: Interoperability

    Marc

    ENC Harmonization Checker - Marc Roesbeke (Flemish Hydrographic Agency) - INSPIRE Conference 2011 (Edinburgh)

  • Jordi ESCRIU

    Thanks Marc, and very interesting.

    From your post, I understand that transition from LAT1 and LAT2 in my example may not be smooth - Is this correct?

    Are ENC cells always vector data sets? - I mean, harmonization problems derived from using different bathymetric references (e.g. different LAT realizations) also affect covarage (raster) data.

  • Marc ROESBEKE

    Jordi,

    Yes, there is no smoothing of depth contours accros the border at sea.

    And the ENC cells are always vector chart data (Worldwide, standard IHO S57 and encrypted IHO S63 vector chart)

    Raster charts formats are: BSB (Maptech Raster File Format), BAG (Bathymetric Attributed Grid) and HCRF (Hydrographic Chart Raster Format)

    Marc

  • Jordi ESCRIU

    Dear All,

    The content of this post reflects that vector Elevation data is useful in the INSPIRE context:

    - In the bathymetry sub-theme, where vector data (referenced to an appropriate vertical reference system, depending on the case) may be provided when raster data is not available.

    - In the land-elevation sub-theme, as a second alternative to raster data provision. However, in this case certain problems may appear when transforming contour from the national systems to the European Vertical Reference System - EVRS (as explained in this discussion topic), and probably the Member States will not opt for using this additional option for data provision.

    Therefore, the original purpose of the post is accomplished - Check the usefulness of such vector data, and I proceed to close this dicussion topic.

    Jordi

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Elevation, Ortho & Grids

Elevation, Ortho & Grids

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