c-squares - a simple,

XML friendly,

query/ display/ exchange format for

representing spatial data at the

metadata level

System concept and development by:

Tony Rees

Divisional Data Centre

CSIRO Marine Research, Australia

examples of current metadata systems ...

new all-CSIRO directoryOBIS, USA (planned)

Metadata versus Data

(Data level)

Data Store 1

(Metadata level)

databases / data warehouses

Data Store 2

offline digital dataoffline nondigital data

Metadata records(text files, database,

or XML format)

dataset descriptions in standard format

metadata query and/or exchange

• Normally a text (ASCII) file, divided into individual metadata fields (e.g. with XML markup or similar)

• Can be searched as plain text (minimum) or, if stored in a database, by more sophisticated methods

• Represents a level of abstraction from the actual data - documents the most useful summary information for potential users, including spatial aspects (e.g. bounding coordinates, location keywords)

• Should function as a stand-alone “surrogate” for the data for discovery and appraisal purposes - frequently decoupled from the actual data (which may or may not be available on-line)

• May have different distribution/access privileges from the actual data.

Some Characteristics of Metadata

<metadata>

<title>Franklin Voyage FR 07/93 CTD Data</title>

<custodianOrg>CSIRO Marine Research</custodianOrg>

(etc. etc.)

<boundingBox>

<northBoundingCoord>-30.0</northBoundingCoord>

<southBoundingCoord>-44.0</southBoundingCoord>

<westBoundingCoord>147.0</westBoundingCoord>

<eastBoundingCoord>173.0</eastBoundingCoord>

</boundingBox>

(etc. etc.)

</metadata>

Current preferred metadata transfer format is XML (ASCII) file, e.g… (excerpt):

“standard” representation of spatial data coverage in a sample metadata record

actual data distribution for the preceding example (CTD data, Franklin voyage FR 07/93)

NB, standard spatial searches (by intersecting or overlapping

rectangles) will produce numerous false “hits” (erroneous values) since most of the bounding rectangle for

this dataset is empty

Data coverage is not always rectangular

- frequently fills a non-rectangular shape (b), follows an edge or contour (c), surrounds an object (e.g. island or continent), (d), is discontinuous (e),

linear/oblique (f), or extremely patchy (g)

(a) (b) (c) (d)

(e) (f) (g)

Why searching using overlapping rectangles is fallible

- for data of types (b) - (g), many searches by overlapping rectangles will produce “false hits”

(a) (b) (c) (d)

(e) (f) (g)

data coverage

bounding rectangle for

dataset

bounding rectangle

specified for search

Note, only one of these plots is at all well represented by its “bounding coordinates” box

Further examples of real data (species distributions)

“c-squares” is a system for improved metadata representation of spatial data which supports reliable spatial queries and provides meaningful visual representations (footprints) of dataset spatial coverage. It is fully transparent, royalty free, and vendor/platform independent, and available for immediate use.

c-squares comprises the following elements:

• a nested set of proposed “standard” spatial resolutions for dataset description (levels 1-3), with all metadata at level 1 potentially interoperable

• a custom, scalable nomenclature for “base level” individual geosquares, building on established standards, in a simple human- and machine- readable format

• a syntax for representing strings of such geosquares in an editable and transportable ASCII format, in particular for use within a XML or similar metadata records

• algorithms for translating any geolocated data between lats/longs and c-square codes, and vice versa, and

• simple connectivity to basic mapping packages, including “freeware” such as the online Xerox PARC mapper.

“c-squares” solution

Example ...

Data “footprint” using bounding coordinates (as held in a

metadata record)

Data “footprint” using c-squares (also in metadata record)

• c-squares level 1 (fully interoperable) operates at 0.5 x 0.5 degree spatial resolution, or some 50-70 kms point-to-point in most regions of the globe

• c-squares levels 2 and 3 (locally interoperable where implemented by users) operate at 0.1 x 0.1 and 0.02 x 0.02 degree (around 10 and 2 kms, respectively) spatial resolution, for higher resolution spatial representation where needed

• c-squares levels 4-unlimited are not formally defined, but can be implemented by users on an “as needs” basis by simple extension from c-squares level 3.

• Examples: c-squares level 1 code: SI5605f (0.5 x 0.5 deg. square)

c-squares level 2 code: SI5605fa (0.1 x 0.1 deg. square)

c-squares level 3 code: SI5605faa (0.02 x 0.02 deg. square)

“c-squares” standard resolutions

• (i) “International Map of The World” (IMW) nomenclature for 4 x 6 deg. rectangles covering the globe (except for tiny regions at the poles) (NA01 - NV60, SA01 - SV60)

• (ii) AUSLIG (Australia) nomenclature for the 16, 1 x 1.5 deg. subunits of each IMW rectangle (e.g. NA0101 - NA0116)

• (iii) c-squares nomenclature for the 6, 0.5 x 0.5 degree square within each AUSLIG rectangle (e.g. NA0101a - NA0101f) (=c-squares level 1)

• (iv) optional further c-squares nomenclature for the 25, 0.1 x 0.1 degree squares within each “base level” c-square (=level 2), and then further subdivision into 25 more, 0.02 x 0.02 degree squares (=level 3).

Basis for “c-squares” nomenclature

6o longitude bands: 01-60, eastwards from -180o (international date line)

e.g.: London (west of Greenwich) is in IMW rectangle NM30

(note, gridlines shown are larger than IMW rectangles)

IMW rectangles nomenclature

4o latitude bands

NA-NV (northwards)

-------SA-SV

(southwards)

IMW 4 x 6 degree rectangles across Australia(= basis for AUSLIG 1:1,000,000 map series)

(inset shows relative size of Australia cf. USA and U.K.)

IMW 4 x 6 degree rectangles (some aggregated) across Canada(= basis for Canadian Geological Survey 1:1,000,000 map series)

1 x 1.5 degree rectangles are numbered from top left corner, -01 to -16 (names are those of the equivalent

AUSLIG 1:250k mapsheets)

AUSLIG 1 x 1.5 degree rectangles nomenclature

e.g. for IMW rectangle SE53:

ROBINSON RIVER - rectangle SE5304

0.5 x 0.5 degree squares: c-squares “base level” nomenclature (= level 1)

SE5304a SE5304b SE5304c

SE5304e SE5304fSE5304d

“level 1” c-squares are approx. 70 x 70 km at this latitude

SYDNEY, Australia - rectangle SE5605

0.5 x 0.5 degree squares: another example (including populated areas)

SE5605a SE5605b SE5605c

SE5605e SE5605fSE5605d

SE5605c SE5605b

SE5605ce

SE5605cg (etc.)

SE5605cdSE5605ccSE5605cbSE5605ca

SE5605cf

0.1 x 0.1 degree (level 2) c-squares: enlargement of two “level 1” c-squares from previous page

each 0.1 x 0.1 degree “level 2” c-square measures approx. 12 x 14 km at this latitude

SE5605cn SE5605cne

SE5605cng (etc.)

SE5605cndSE5605cncSE5605cnbSE5605cna

SE5605cnf

0.02 x 0.02 degree (level 3) c-squares are further subdivision of level 2 squares:

each 0.02 x 0.02 degree “level 3” c-square measures approx. 2 x 2.5 km at this latitude

• 0.5 x 0.5 degree squares can be aggregated for display purposes at a range of sizes (squares or rectangles) to suit user’s needs, e.g. to 1 x 1, 1.5 x 1, 2.5 x 2.5, 4 x 4, 5 x 5 , 6 x 4, 10 x 10 degree units

• each c-square “knows” the name of its parents in the hierarchical system (makes searching/matching very simple, at a range of scales, using simple text-based queries)

• repetitive information can be compacted, e.g. “ SK5501* ” would indicate all 6 base level c-squares within AUSLIG rectangle SK5501; “ SK55* ” would indicate all 96 base-level c-squares within IMW rectangle SK55.

Additional features of c-squares nomenclature

<metadata>

<title>Franklin Voyage FR 07/93 CTD Data</title>

<custodianOrg>CSIRO Marine Research</custodianOrg>

(etc. etc.)

<boundingBox>

<northBoundingCoord>-30.0</northBoundingCoord> <southBoundingCoord>-44.0</southBoundingCoord> <westBoundingCoord>147.0</westBoundingCoord> <eastBoundingCoord>173.0</eastBoundingCoord> </boundingBox>

<csquares>SK5516a|SK5514c|SK5614c|SK5614b|SK5713a|SK5714b|SK5613b|SK5714a|SK5614a|SK5814b|SK5814c|SK5813b|SK5714b|SK5714c|SK5716c|SK5516b|SK5814a|SK5716a|SK5613a|SK5616b|SK5713c|SJ5904a|SI5912a|SI5908d|SI5907f|SI5903f|SI5903c|SH5916a|SH5611b|SH5911a|SH5909c|SH5911b|SH5910c|SH5910a|SH5810b|SH5810a|SH5812c|SH5811c|SH5812a|SH5709b|SH5909a|SH5611c|SH5812b|SH5709c|SH5710b|SH5709a|SH5809a|SH5612b|SH5712c|SH5712b|SH5809b|SH5811b|SH5612a|SH5811a|SH5711c|SH5611a|SH5712a|SH5710c|SH5711a</csquares> (etc. etc.)

</metadata>

Syntax for c-squares strings (e.g as xml)

• user consults an atlas or mapsheet “marked up” with the c-square code, and enters them manually

• user enters coordinates of a data point into an on-line conversion utility, and software returns the relevant c-square code

• user draws a polygon around data extent/s using a digitising system, and software calculates the relevant c-square codes

• a simple, automated routine is run over the individual base data points and generates the relevant c-square codes (this can result in considerable data reduction as multiple records in a single square are ignored)

Options for c-squares encoding of spatial data

• A simple function is used to transform the c-squares string into a list of centre points of relevant gridsquares, e.g. giving the following:

• -43.25,148.75;-43.25,146.75;-43.25,152.75;-43.25,152.25;-43.25,156.25;-43.25,158.25;-43.25,150.75;-43.25,157.75;-43.25,151.75;-43.25,164.25;-43.25,164.75;-43.25,162.75;-43.25,158.75;-43.25,161.75;-43.25,149.25;-43.25,163.75;-43.25,160.75;-43.25,150.25;-43.25,155.25;-43.25,157.25;-36.25,172.75;-34.25,172.75;-33.75,172.75;-33.75,172.25;-32.75,172.25;-32.25,172.25;-31.25,172.75;-30.25,153.75;-30.25,171.25;-30.25,169.25;-30.25,171.75;-30.25,170.75;-30.25,169.75;-30.25,164.25;-30.25,163.75;-30.25,167.75;-30.25,166.25;-30.25,166.75;-30.25,156.75;-30.25,168.25;-30.25,154.25;-30.25,167.25;-30.25,157.25;-30.25,158.25;-30.25,156.25;-30.25,162.25;-30.25,155.25;-30.25,161.75;-30.25,161.25;-30.25,162.75;-30.25,165.75;-30.25,154.75;-30.25,165.25;-30.25,160.25;-30.25,153.25;-30.25,160.75;-30.25,158.75;-30.25,159.25

(note, this is more verbose and also less human-intelligible than the c-squares version)

• This string can then be sent to a custom mapping package, or free online web service such as Xerox PARC mapper, to produce an interactive map e.g. as follows:

Decoding of c-squares strings

Sample c-squares enabled metadata record, with pre-formatted links to Xerox PARC mapper:

level 1 c-squares displayed at 0.5 x 0.5 degree (native) resolution

level 1 c-squares displayed at 2 x 2 degree (aggregated) resolution

level 1 c-squares displayed at 1 x 1 degree (aggregated) resolution

• any metadata record or similar summary information describing spatial data, at scales from c. 0.02 degrees (2 km) upwards (or with progressively finer resolution as user desires)

• subject to encouraging user feedback, c-squares may be recommended to CSIRO Australia, ANZLIC, and other relevant bodies as an optional metadata element

• the “c-squares” name and concept remains the property of CSIRO Marine Research and may be further developed, but the system is available for use royalty-free and without restriction as a contribution to global metadata exchange and usefulness for describing spatial data.

c-squares applicability:

Further information is available from

Tony Rees, CSIRO Marine Research, Australia

(Tony.Rees@csiro.au)

or via the c-squares website:

http://www.marine.csiro.au/csquares/