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How to implement the concept of mosaicking

Introduction

This discussion topic was originated in the following thread dedicated to publish Experiences on encoding of Elevation and Orthoimagery coverages.

After the identification of the issue some dicussion took place in the topic How to implement tiling / model mosaic elements, coverages and coverage aggregations in GMLCOV files.

However the topic was partially mixed with other ones, which lead to some misunderstandings.

During the Workshop "Implementation and potential of INSPIRE coverage data and WCS" (30th September 2016 - INSPIRE Conference 2016, Barcelona) there was the opportunity to meet with the community of Thematic Cluster #3, discuss face-to-face and clearly delimitate the most important issues on encoding of coverages - being this topic one of them.

Summary

The concept of mosaicking as used in the INSPIRE TG on OI is related to data produced in this thematic domain. The best way to explain it is by means of an example. Orthoimages (i.e. OI coverages), namely orthophotos, are mosaics composed of a contiguous set of irregular image areas (delimited by seamlines), whose radiometric values come from different orthorectified scenes (with the same grid and the same range type). Each of these scenes is the result of orthorectifying an image (which is often in a conic projection) captured by means of an airborne sensor, at a specific date of acquisition. Such source images partially overlap each other, providing stereoscopic capabilities in the overlapping areas between 2 contiguous scenes. This schema is systematically repeated to recover the whole area of interest. Therefore, the orthoimage coverage constitutes a “mosaic” composed of selected areas from the mentioned orthorectified scenes. And the radiometric values corresponding to each of these areas are the result of resampling the ones from the original source image through an orthorectification process - i.e. derived from the values captured by the sensor in a specific date.

Figure – Mosaicked orthoimage with seamlines.

Figure – Mosaicked orthoimage with seamlines.

Hence, the orthoimage mosaic (‘Mosaic’ feature type from INSPIRE TG on OI) supports the provision of the acquisition dates related to each of the orthorectified scenes participating in the orthoimage coverage – i.e. related to the range set values of different areas of the mosaicked coverage.

There is also implementation experience available – at least from IGN-Spain - on how to model the concept of mosaicking using the current implementation standard (GMLCOV/CIS).

Usefulness of this information is clear at INSPIRE level. However, there might be different ways to store this temporal ancillary information – To be further explored.

Modelling this concept as it is proposed by INSPIRE OI adds complexity. Additionally, it also constitutes an INSPIRE Extension. As justified before, it will be ignored by standard WCS interfaces. Besides, the maintenance of “mosaicking” is as costly as “coverage aggregation” is.

Pending discussion

It is suggested, therefore, to provide guidelines for including this "mosaicking" information.

The coverage metadata hook could be used in case of delivering data by means of WCS (to be further analysed how to do it).

Other pending issues which came up during the discussion on this topic in the workshop:

  • The concept of “mosaicking” is only proposed in the INSPIRE TG on OI, while it is also useful (but not currently modelled) in the TG on EL. It might be also applicable to other INSPIRE themes, like Land Cover. Therefore, there is a lack of cross-theme harmonization that should be addressed.
  • It would be also interesting to explore the potential of coverage temporal analysis, e.g. modelling this information in the INSPIRE coverage models (such as representing image timeseries as 3-D datacubes for easy slicing, etc).

An online webinar will be scheduled in early 2017 to discuss this specific implementation aspect.

Let's discuss on it!

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