Clarify some of the description of input[type=zoom], add fragment id for

linking.

Author: prushforth

.gitignore

@@ -2,4 +2,5 @@ nbproject/private/private.properties
 nbproject/project.properties
 nbproject/project.xml
 nbproject/private/private.xml
-/schema/mapml_validation.xpr
+/schema/mapml_validation.xpr
+/nbproject/private/

spec/index.html

@@ -802,8 +802,10 @@ <h5 id="OTHERPROJECTIONS">Other coordinate reference systems</h5>
         <h4 id="scale">4.1.4 Scale / Resolution / Zoom levels</h4>
         <p>Scale is an essential characteristic of all geospatial information. The scale at which information is used dictates the model used for its representation. Geospatial information cannot be meaningfully combined without consideration given to its scale. Entire cities are represented on 'small scale' maps as dots, whereas on another large scale map, a single city could have many thousands of individual features in its depiction.</p>
         <p>Not only is scale important in the portrayal of maps, it is also essential when defining the model of underlying feature data.</p>
-        <p>Map projections distort the surface of the earth in ways which suit the objectives of the projection definition. As a result of that distortion, the scale and the resolution of maps in that projection vary as a function of location. The resolutions specified by this document are only valid in defined locations. For example, in WGS84 and OSMTILE the defined resolutions are valid only along the equator. "Zoom" levels are integer values corresponding to the index of the resolution value defined above, and represent numeric proxy for a scale, at the defined location(s). For example, the OSMTILE TCRS, being based internally on the Spherical Web Mercator projection (EPSG::3857), allows us to reasonably portray a large portion of the Earth's surface, requiring relatively simple and fast math to convert to and from WGS84. Additionally, it enables the simple tiling system that has become a <i>de facto</i> standard. As such, it was deemed to have appropriate properties for a globally useful projection on the Web.</p>
-        <p>Map documents encoded in MapML have a defined scale, indirectly identified by zoom level and extent. Thus, two MapML documents in the same TCRS and zoom level should be interoperable to the degree that they can be automatically and visually related. The zoom level of a MapML document is often identified as the value of the <code>value</code> attribute of the <code>input[@type=zoom]</code>. In any case, the zoom level of any MapML document SHOULD be present in document metadata as a <code>meta</code> element e.g. &lt;meta name="zoom" content="0"/&gt;.  In the case where a MapML document contains an <code>extent</code> element, an <code>input@type=zoom</code> MUST be present which controls how the zoom is transmitted from client to server. Where that input has a non-empty <code>input@value</code> attribute, in case of discrepancy between  the <code>meta[@name=zoom]/@content</code> and the <code>input[@type=zoom]/@value</code>, the latter zoom value SHALL be taken to be correct, EXCEPT in the case where there may exist more than one <code>input[@type=zoom]</code> and their <code>value</code> attributes do not agree, in which case the value of <code>meta[@name=zoom]/@content</code> shall be taken to be correct.</p>
+                <p>Map projections distort the surface of the earth in ways which suit the objectives of the projection definition. As a result of that distortion, the scale and the resolution of maps in that projection vary as a function of location. The resolutions specified by this document are only valid in defined locations. For example, in WGS84 and OSMTILE the defined resolutions are valid only along the equator. "Zoom" levels are integer values corresponding to the index of the resolution value defined above, and represent numeric proxy for a scale, at the defined location(s). For example, the OSMTILE TCRS, being based internally on the Spherical Web Mercator projection (EPSG::3857), allows us to reasonably portray a large portion of the Earth's surface, requiring relatively simple and fast math to convert to and from spherical latitude/longitude coordinates. Additionally, it enables the simple tiling system known as OSMTILE in this specification, that has become a <i>de facto</i> standard. As such, it was deemed to have appropriate properties for a globally useful projection on the Web.</p>
+                <p id="zoom">In MapML documents, scale is indirectly represented by zoom level, and varies across any rendered map extent. Two MapML documents in the same TCRS and zoom level should be interoperable to the degree that they can be automatically and visually related (overlayed). The zoom level of a MapML document is often represented by the value of the <code>value</code> attribute of the <code>input[@type=zoom]</code>. By associating a specific zoom <code>value</code> to the input, we are able to compute the spatial bounds of a <code>link</code> template by means of <code>min</code>, <code>max</code> and <code>axis</code> values of other <code>input[@type=location]</code> elements associated to the same <code>link</code> template element with which the <code>input[@type=zoom]</code> is associated. 
+                  
+                  In any case, the zoom level of any MapML document MAY be present in document metadata as a <code>meta</code> element e.g. &lt;meta name="zoom" content="0"/&gt;.  In the case where a MapML document contains an <code>extent</code> element, an <code>input@type=zoom</code> MUST be present which controls how the zoom is transmitted from client to server. Where that input has a non-empty <code>input@value</code> attribute, in case of discrepancy between  the <code>meta[@name=zoom]/@content</code> and the <code>input[@type=zoom]/@value</code>, the latter zoom value SHALL be taken to be correct, EXCEPT in the case where there may exist more than one <code>input[@type=zoom]</code> and their <code>value</code> attributes do not agree, in which case the value of <code>meta[@name=zoom]/@content</code> shall be taken to be correct.</p>
         <h4 id="extent-semantics">4.1.5 Extents</h4>
         <p>A MapML document is a representation of a defined portion of a two dimensional map area. In MapML, this is called an 'extent' (see below), the dimensions of which can be described by the bounding axis minimum and maximum values of the specified TCRS, or in one of its associated coordinate reference systems (PCRS,GCRS,Tilematrix,etc.).
           The bounds of an extent SHOULD be specified in a MapML document, by the use of coordinates and/or coordinate axes whose coordinate reference system is identified or implied by the <code>extent/@units</code> value. If no <code>extent/@units</code> element or attribute is present, the <code>meta[@name=tcrs]</code> element MAY describe or identify the TCRS of the MapML document.