High resolution alyer HRL, imperviousness – imperviousness density report 2015

Abstract

A nation-wide exercise was carried out to analyse the effective built-up areas using in-situ imagery data and the Degree of Imperviousness layers (IMD 2015) provided for verification. The IMD 2015 data layer was superimposed onto the in-situ data; ortho-photos and the CLC layer. Subsequently a thematic map was prepared whereby the main areas to be interpreted as Built-up were highlighted, namely the areas with pixel values of 30% to 100% Imperviousness.

In Malta’s case it was deemed best to apply a complete coverage methodology rather than verifying specific area samples. Hence each area marked as “Built-up” was verified visually utilising in-situ Orthoimagery and the CLC databases (for the reference years 2012 and 2018) and all the major errors identified and categorised as “Commission” or “Omission” error types as per guidelines provided for the verification process. When considering the physical area of the Maltese Islands and the land cover characteristics of the built-up areas, applying this method of photo interpretation, resulted in a more adequate verification of the HRL layer giving us a more detailed result.

Overall, the Impervious HRL layer provides a generalised view compared to the level of detail that is normally utilized locally (1:1000). Even so, the detail provided in the IMD2015 layer can still be deemed very good for the parameters/requirements of this verification exercise. During this verification stage, the majority of the errors were “Omissions” particularly in areas where the development was isolated or their shape was elongated and hence forming narrow strips of built-up areas. The latter issue was particularly evident in the island of Gozo and in some parts of the north west of the island of Malta. A significant number of Omission cases related to greenhouses, cemeteries and sports and recreation areas which were erroneously interpreted as non-impervious.