Lack of best practice standard

The [MODS v3 specification|http://www.loc.gov/standards/mods/userguide/subject.html#coordinates] is vague in how coordinates should be formatted, and in practice there doesn't seem to be a standard format. It claims:

{quote}
One or more statements may be supplied. If one is supplied, it is a point (i.e., a single location); if two, it is a line; if more than two, it is an n-sided polygon where n=number of coordinates assigned. No three points should be co-linear, and coordinates should be supplied in polygon-traversal order.
{quote}

We have some [MODS Guidelines|https://consul.stanford.edu/display/chimera/MODS+display+rules] here in DLSS but they do not have a normative recommendation for coordinates other than [MARC 034|http://www.loc.gov/marc/bibliographic/concise/bd034.html] and [MARC 255|http://www.loc.gov/marc/bibliographic/concise/bd255.html], which all have multiple representations.

Design Alternatives

Goals:
* Include human\- and machine-readable encodings for geospatial coordinates for MODS without requiring a _geoMetadata_ datastream (e.g., paper maps vs. GIS data)
* Clean, simple for MODS display logic
* Pass XML validation for [MODS 3.4\+ schema|http://www.loc.gov/standards/mods/mods.xsd]
* Support point and bounding box coordinates for geospatial indexing, in multiple projections
* Compatible with MARC034 and MARC255 formats

MODS Geo Extension

The human-readable data would be in MODS as usual in subject/cartographics/coordinates, but the machine-readable data would be in a MODS geo extension using an existing geospatial standard.

Using GML and RDF we can support arbitrary projections for bounding boxes, and using Dublin Core we can define spatial facets like the format (e.g., a Shapefile) and type (e.g., a Dataset with point data), and associated place names.

{code:xml|title=Extension using machine-readable OGC GML and RDF}
<mods>
...
<titleInfo>
<title>Oil and Gas Fields in the United States, 2011</title>
</titleInfo>
...
<subject>
<cartographics>
<scale>Scale not given.</scale>
<projection>North American Datum 1983 (NAD83)</projection>
<coordinates>(W 151°28ʹ46ʺ--W 78°5ʹ6ʺ/N 69°25ʹ57ʺ--N 26°4ʹ18ʺ)</coordinates>
</cartographics>
</subject>
...

<extension xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" displayLabel="geo">
<rdf:RDF xmlns:gml="http://www.opengis.net/gml/3.2/" xmlns:dc="http://purl.org/dc/elements/1.1/">
<rdf:Description rdf:about="http://purl.stanford.edu/cs838pw3418">
<dc:format>application/x-esri-shapefile</dc:format>
<dc:type>Dataset#point</dc:type>
<gml:boundedBy>
<gml:Envelope gml:srsName="EPSG:4269">
<gml:lowerCorner>-151.479444 26.071745</gml:lowerCorner>
<gml:upperCorner>-78.085007 69.4325</gml:upperCorner>
</gml:Envelope>
</gml:boundedBy>
<dc:coverage rdf:resource="http://geonames.org/6252001" dc:language="eng" dc:title="United States"/>
<dc:coverage rdf:resource="http://geonames.org/5332921" dc:language="eng" dc:title="California"/>
</rdf:Description>
</rdf:RDF>
</extension>
</mods>
{code}

the bounding box can optionally include the valid time period:

{code:xml}
<gml:boundedBy>
<gml:EnvelopeWithTimePeriod gml:srsName="EPSG:4269">
<gml:lowerCorner>-151.479444 26.071745</gml:lowerCorner>
<gml:upperCorner>-78.085007 69.4325</gml:upperCorner>
<gml:beginPosition>2008</gml:beginPosition>
<gml:endPosition>2008</gml:endPosition>
</gml:EnvelopeWithTimePeriod>
</gml:boundedBy>
{code}

The placenames are a 0..n mapping:

Schema

!sample_schema.png|width=800!

RDF Triples for Format and Type

!servlet_3266409606106675285.png|width=800!

RDF Triples for Bounding Box

!servlet_5082135491669855390.png|width=800!

RDF Triples for Placenames

!servlet_6294733788115196033.png|width=800!

Example encodings for coordinate data

There are several open standards for encoding bounding boxes:

* [ISO 19139|https://consul.stanford.edu/display/SULAIRGIS/Metadata+Creation+for+GIS+Data]
* [OGC WKT|http://portal.opengeospatial.org/files/?artifact_id=25355]
* [OGC WMS|http://portal.opengeospatial.org/files/?artifact_id=14416]
* [OGC GML|http://portal.opengeospatial.org/files/?artifact_id=20509]
* [GeoRSS Simple|http://georss.org/simple]
* [MARC 034|http://www.loc.gov/marc/bibliographic/bd034.html]
* [MARC 255|http://www.loc.gov/marc/bibliographic/bd255.html]

The XML namespaces are:

* xmlns:wms="http://www.opengis.net/wms"
* xmlns:gml="http://www.opengis.net/gml/3.2"
* xmlns:georss="http://www.georss.org/georss"

*Check on how to do datum encoding*

{code:xml|title=ISO 19139}
<gmd:EX_GeographicBoundingBox>
<gmd:westBoundLongitude>
<gco:Decimal>-151.479444</gco:Decimal>
</gmd:westBoundLongitude>
<gmd:eastBoundLongitude>
<gco:Decimal>-78.085007</gco:Decimal>
</gmd:eastBoundLongitude>
<gmd:southBoundLatitude>
<gco:Decimal>26.071745</gco:Decimal>
</gmd:southBoundLatitude>
<gmd:northBoundLatitude>
<gco:Decimal>69.4325</gco:Decimal>
</gmd:northBoundLatitude>
</gmd:EX_GeographicBoundingBox>
{code}

*Check on how to do datum encoding*

{code:title=OGC WKT}
POINT(-151.479444 26.071745)

POLYGON((-151.479444 26.071745,
-151.479444 69.4325,
-78.085007 69.4325,
-78.085007 26.071745,
-151.479444 26.071745))
{code}

{code:xml|title=OGC WMS}
<wms:BoundingBox wms:CRS="EPSG:4269"
wms:minx="-151.479444" wms:miny="26.071745"
wms:maxx="-78.085007" wms:maxy="69.4325"/>
{code}

{code:xml|title=OGC GML}
<gml:Point gml:srsName="EPSG:4269">-151.479444 26.071745</gml:Point>

<gml:Envelope gml:srsName="EPSG:4269">
<gml:lowerCorner>-151.479444 26.071745</gml:lowerCorner>
<gml:upperCorner>-78.085007 69.4325</gml:upperCorner>
</gml:Envelope>
{code}

GeoRSS requires all coordinates to be in WGS84 projection.

*Do projection*

{code:xml|title=GeoRSS Simple}
<georss:point>26.071745 -151.479444</georss:point>

<georss:box>
26.071745 -151.479444 69.4325 -78.085007
</georss:box>
{code}

*Is there a projection required?*

{code:title=MARC 034}
$d W1512846 $e W0780506 $f N0692557 $g N0260418
{code}

MARC 034:

{quote}
For digital items, the coordinates can represent a bounding rectangle, the outline of the area covered and/or the outline of an interior area not covered. [] There should be an 034 field corresponding to each 255 field in a record.
{quote}

{code:title=MARC 255}
$b North American Datum 1983 (NAD83)
$c (W 151°28ʹ46ʺ--W 78°5ʹ6ʺ/N 69°25ʹ57ʺ--N 26°4ʹ18ʺ)

{code}

MARC 255$c:

{quote}
Coordinates are recorded in the order of westernmost longitude, easternmost longitude, northernmost latitude, and southernmost latitude. In records formulated according to ISBD principles, subfield _$c_ data are enclosed in parentheses. The two longitude statements and the two latitude statements are each separated by two hyphens ( -- ). The longitude is separated from the latitude by a slash ( / ).
{quote}

XSLT on ISO 19139

Here is an [XSLT implementation|https://github.com/sul-dlss/dor-services/blob/geoMetadata/lib/dor/datastreams/geo2mods.xsl] for converting into the MODS formats shown above.
See XSLT's [format-number|http://www.w3.org/TR/1999/REC-xslt-19991116#format-number].

Subject Cartographics with altRepGroup

The MODS subject attribute _altRepGroup_ groups records into a single entity.
The MODS display logic treats the subjects belonging to the same _altRepGroup_ to be the same content, and chooses one to use.

{quote}
The MODS schema 3.4 defined a new attribute @altRepGroup (with string values) to be used with top-level elements. This attribute is used to link alternate representations of the same element content, such as different scripts, transliterations, and translations. The same attribute value is applied to each of the element instances to be linked.
{quote}

This is a general example of the usage:

{code:xml|title=Subject with altRepGroup}
<mods>
<subject altRepGroup="123" usage="primary display">
<topic>
Cats
</topic>
</subject>

<subject altRepGroup="123">
<topic>
Felis silvestris catus
</topic>
</subject>
</mods>
{code}

A secondary subject in the same altRepGroup could be used to encode coordinates by convention. In this example, we use a OGC WKT format to encode the coordinates.

{code:xml|title=Subject with altRepGroup and example geospatial coordinates}
<mods>
<subject altRepGroup="789" usage="primary display">
<cartographics>
<projection>
North American Datum 1983 (NAD83)
</projection>
<coordinates>
(W 151°28ʹ46ʺ--W 78°5ʹ6ʺ/N 69°25ʹ57ʺ--N 26°4ʹ18ʺ)
</coordinates>
</cartographics>
</subject>

<subject altRepGroup="789">
<cartographics authority="local:geohydra">
<projection>
North American Datum 1983 (NAD83)
</projection>
<coordinates>
POLYGON(-151.479444 26.071745,
-151.479444 69.4325,
-78.085007 69.4325,
-78.085007 26.071745,
-151.479444 26.071745)
</coordinates>
</cartographics>
</subject>
</mods>
{code}

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