| Summary: | The content at the end of any hyperlink is subject to two phenomena: the link may break (<i>Link Rot</i>) or the content at the end of the link may no longer be the same as it was when it was created (<i>Content Drift</i>). <i>Reference Rot</i> denotes the combination of both effects. Spatial metadata records rely on hyperlinks for indicating the location of the resources they describe. Therefore, they are also subject to <i>Reference Rot</i>. This paper evaluates the presence of <i>Reference Rot</i> and its impact on the 22,738 distribution URIs of 18,054 metadata records from 26 European INSPIRE spatial data catalogues. Our <i>Link Rot</i> checking method detects broken links while considering the specific requirements of spatial data services. Our <i>Content Drift</i> checking method uses the data format as an indicator. It compares the data formats declared in the metadata with the actual data types returned by the hyperlinks. Findings show that <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>10.41</mn><mo>%</mo></mrow></semantics></math></inline-formula> of the distribution URIs suffer from <i>Link Rot</i> and at least <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>6.21</mn><mo>%</mo></mrow></semantics></math></inline-formula> of records suffer from <i>Content Drift</i> (do not declare its distribution types correctly). Additionally, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>14.94</mn><mo>%</mo></mrow></semantics></math></inline-formula> of metadata records only contain intermediate HTML web pages as distribution URIs and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>31.37</mn><mo>%</mo></mrow></semantics></math></inline-formula> contain at least one HTML web page; thus, they cannot be accessed or checked directly.
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