feat: complete Wallonia land cover and terrain sources

docs/DATA_SPECIFICATION.md

@@ -317,6 +317,19 @@ Area metrics group forest/tree cover as `{8,9,80,90}`, water as `{5}`,
 artificial cover as `{1,2,3}`, rotating herbaceous cover as `{6}`, continuous
 herbaceous cover as `{7}` and bare soil as `{4}`.
 
+The 2018 WALOUS edition additionally retains the original stacked source codes
+and the complete official `Classe vue` crosswalk in provenance. Code `62`
+(greenhouses) is normalized to canonical construction class `2`; this and the
+2018 production-method difference must be displayed as a temporal comparison
+limitation. Source value `0` is explicit implicit-background/nodata and is
+never counted as a land-cover class.
+
+The Wallonia MNT source retains EPSG:3812, native 1 m resolution, Float terrain
+values, source checksum, acquisition period 2021-02-19/2022-03-05 and vertical
+reference EPSG:5710 (DNG). Bounded derivatives use Float32, nodata `-9999`,
+bilinear resampling and the selected Area as an exact mask. DNG and TAW are not
+interchanged.
+
 ### Hydrological station observations
 
 Waterinfo observations are persisted as EPSG:4326 Point features, one station