Update README.md

README.md

@@ -14,3 +14,41 @@ As data ingest is tightly connected with metadata management, use of data, etc.,
 
 - [resource-metadata](https://github.com/FAIRiCUBE/resource-metadata): in addition to the issues providing metadata for resources, also used to discuss technical details on resource metadata
 - [Fairicube Hub](https://github.com/FAIRiCUBE/FAIRiCUBE-Hub-issue-tracker): for general FAIRiCUBE topics
+
+-----
+
+# Range Type
+
+(this will go on a separate page later)
+
+Responding to requirements phrased in the project the range type in coverages has been extended in rasdaman. As this is a new feature it is documented here to ease understanding and use.
+
+Of particular interest are the components:
+
+- field name: the record component name under which range values are known and can be extracted, such as `$c.red` in WCPS; this must follow the usual syntax conventions of program variable names
+- definition: the technical data type (int, float, etc.), expressed through OGC URIs as OGC requires
+- description: a human-readable text explaining the meaning
+- nil values, as pairs of reason (some text) and a value (taken from the data type definition, see above)
+- unit of measure: OGC has no guidance on this; earlier UCUM was used, but it appears inconvenient. In Fairicube QUDT seems preferred - more to come once agreed, stay tuned.
+
+In XML, the complete range type structure is as in the following example:
+
+```
+<gmlcov:rangeType>
+    <swe:DataRecord>
+        <swe:field name="red">
+            <swe:Quantity definition="http://www.opengis.net/def/dataType/OGC/0/unsignedByte">
+                <swe:label>red</swe:label>
+                <swe:description>Band 1 description</swe:description>
+                <swe:nilValues>
+                    <swe:NilValues>
+                        <swe:nilValue reason="I don't know">25</swe:nilValue>
+                    </swe:NilValues>
+                </swe:nilValues>
+                <swe:uom code="10^0"/>
+            </swe:Quantity>
+        </swe:field>
+        <swe:field name="...">...</swe:field>
+    </swe:DataRecord>
+</gmlcov:rangeType>
+```