11. Cheatsheets

11.1. WCS

The OGC Web Coverage Service (WCS) standard defines support for modeling and retrieval of geospatial data as coverages (e.g. sensor, image, or statistics data).

WCS consists of a Core specification for basic operation support with regards to coverage discovery and retreival, and various Extension specifications for optional capabilities that a service could provide on offered coverage objects.

11.1.1. Core

The Core specification is agnostic of implementation details, hence, access syntax and mechanics are defined by protocol extensions: KVP/GET, XML/POST, and XML/SOAP. Rasdaman supports all three, but further on the examples are in KVP/GET exclusively, as it is the most straightforward way for constructing requests by appending a standard query string to the service endpoint URL. Commonly, for all operations the KVP/GET request will look as follows:

http(s)://<endpoint url>?service=WCS
                        &version=2.0.1
                        &request=<operation>
                        &...

Three fundamental operations are defined by the Core:

11.1.2. Updating

The Transaction extension (WCS-T) specifies the following operations for constructing, maintenance, and removal of coverages on a server: InsertCoverage, UpdateCoverage, and DeleteCoverage.

Rasdaman provides the wcst_import tool to simplify the ingestion of data into analysis-ready coverages (aka datacubes) by generating WCS-T requests as instructed by a simple configuration file.

11.1.3. Processing

The Processing extension enables advanced analytics on coverages through WCPS queries. The request format is as follows:

http(s)://<endpoint url>?service=WCS&version=2.0.1&request=ProcessCoverages
                        &query=<wcps query>

E.g, calculate the average on the subset from the previous GetCoverage example:

11.1.4. Range subsetting

The cell values of some coverages consist of multiple components (also known as ranges, bands, channels, fields, attributes). The Range subsetting extension specifies the extraction and/or recombination in possibly different order of one or more bands. This is done by listing the wanted bands or band intervals; e.g AverageChlorophyllScaled has Blue, Green, and Red bands and the following recombines them into a Red, Green, Blue order:

11.1.5. Scaling

Scaling up or down is a common operation supported by the Scaling extension. An additional GetCoverage parameter indicates the scale factor in several possible ways: as a single number applying to all axes, multiple numbers applying to individual axes, full target scale domain, or per-axis target scale domains. E.g. a single factor to downscale all axes by 4x:

11.1.7. Interpolation

Scaling or reprojection can be performed with various interpolation methods as enabled by the Interpolation extension:

Rasdaman supports several interpolations as documented here.

11.2. WCPS

The OGC Web Coverage Processing Service (WCPS) standard defines a protocol-independent declarative query language for the extraction, processing, and analysis of multi-dimensional coverages representing sensor, image, or statistics data.

The overall execution model of WCPS queries is similar to XQuery FLOWR:

for $covIter1 in (covName, ...),
    $covIter2 in (covName, ...),
    ...
let $aliasVar1 := covExpr,
    $aliasVar2 := covExpr,
    ...
where booleanExpr
return processingExpr

Any coverage listed in the WCS GetCapabilities response can be used in place of covName. Multiple $covIter essentially translate to nested loops. For each iteration, the return clause is evaluated if the result of the where clause is true. Coverage iterators and alias variables can be freely used in where / return expressions.

Conforming WCPS queries can be submitted to rasdaman as WCS ProcessCoverages requests, e.g:

http://localhost:8080/rasdaman/ows?service=WCS&version=2.0.1
    &request=ProcessCoverages
    &query=for $covIter in (covName) ...

The WCS-client deployed with every rasdaman installation provides a convenient console for interactively writing and executing WCPS queries: open http://localhost:8080/rasdaman/ows in your Web browser and proceed to the ProcessCoverages tab.

Operations can be categorized by the type of data they result in: scalar, coverage, or metadata.

11.2.1. Scalar operations

  • Standard operations applied on scalar operands return scalar results:

    Operation category Operations
    Arithmetic +  -  *  /  abs  round
    Exponential exp  log  ln  pow  sqrt
    Trigonometric
    sin  cos  tan  sinh  cosh  tanh
    arcsin  arccos  arctan
    Comparison >  <  >=  <=  =  !=
    Logical and  or  xor  not  bit  overlay
    Select field from multiband value .
    Create multiband value { bandName: value; ..., bandName: value }
    Type casting

    (baseType) value

    where baseType is one of: boolean,
    [unsigned] char / short / int / long,
    float, double, complex, complex2
  • Aggregation operations summarize coverages into a scalar value.

    Aggregation type Function / Expression
    Of numeric coverages avg, add, min, max
    Of boolean coverages
    count number of true values;
    some/all = true if some/all values are true
    General condenser
    condense op
    over $iterVar axis(lo:hi), …
    [ where boolScalarExpr ]
    using scalarExpr

    The general condenser aggregates values across an iteration domain with a condenser operation op (one of +, *, max, min, and, or or). For each coordinate in the iteration domain defined by the over clause, the scalar expression in the using clause is evaluated and added to the final aggregated result; the optional where clause allows to filter values from the aggregation.

11.2.2. Coverage operations

  • Standard operations applied on coverage (or mixed coverage and scalar) operands return coverage results. The operation is applied pair-wise on each cell from the coverage operands, or on the scalars and each cell from the coverage in case some of the operands are scalars. All coverage operands must have matching domains and CRS.

  • Subsetting allows to select a part of a coverage (or crop it to a smaller domain):

    covExpr[ axis1(lo:hi), axis2(slice), axis3:crs(...), ... ]
    
    1. axis1 in the result is reduced to span from coordinate lo to hi. Either or both lo and hi can be indicated as *, corresponding to the minimum or maximum bound of that axis.
    2. axis2 is restricted to the exact slice coordinate and removed from the result.
    3. axis3 is subsetted in coordinates specified in the given crs. By default coordinates must be given in the native CRS of C.
  • Extend is similar to subsetting but can be used to enlarge a coverage with null values as well, i.e. lo and hi can extend beyond the min/max bounds of a particular axis; only trimming is possible:

    extend( covExpr, { axis1(lo:hi), axis2:crs(lo:hi), ... } )
    
  • Scale is like extend but it resamples the current coverage values to fit the new domain:

    scale( covExpr, { axis1(lo:hi), axis2:crs(lo:hi), ... } )
    
  • Reproject allows to change the CRS of the coverage:

    crsTransform( covExpr, { axis1:crs1, axis2:crs2, ... } )
    
  • Conditional evaluation is possible with the switch statement:

    switch
      case boolCovExpr return covExpr
      case boolCovExpr return covExpr
      ...
      default return covExpr
    
  • General coverage constructor allows to create a coverage given a domain, where for each coordinate in the domain the value is dynamically calculated from a value expression which potentially references the iterator variables:

    coverage covName
    over $iterVar axis(lo:hi), ...
    values scalarExpr
    
  • General condenser on coverages is same as the scalar general condenser, except that in the using clause we have a coverage expression. The coverage values produced in each iteration are cell-wise aggregated into a single result coverage.

    condense op
    over $iterVar axis(lo:hi), ...
    [ where boolScalarExpr ]
    values covExpr
    
  • Encode allows to export coverages in a specified data format, e.g:

    encode(covExpr, "image/jpeg")
    

11.2.3. Metadata operations

Several functions allow to extract metadata information about a coverage C:

Metadata function Result
imageCrsDomain(C, a) Grid (lo, hi) bounds for axis a.
domain(C, a, c) Geo (lo, hi) bounds for axis a in CRS c.
crsSet(C) Set of CRS identifiers.
nullSet(C) Set of null values.

11.3. WMS

The OGC Web Map Service (WMS) standard defines map portrayal on geo-spatial data. In rasdaman, a WMS service can be enabled on any coverage, including 3-D or higher dimensional; the latest 1.3.0 version is supported.

rasdaman supports two operations: GetCapabilities, GetMap from the standard. We will not go into the details, as users do not normally hand-write WMS requests, but let a client tool or library generate them instead. Please check the Clients section for some examples.

11.4. Clients

11.4.1. Rasdaman WSClient

WSClient is a web-client application to interact with WCS (version 2.0.1) and WMS (version 1.3.0) compliant servers. Once rasdaman is installed it is usually accessible at http://localhost:8080/rasdaman/ows; a publicly accessible example is available at http://ows.rasdaman.org/rasdaman/ows. The client has three main tabs: OGC Web Coverage Service (WCS), OGC Web Map Service (WMS) and Admin. Further on, the functionality in each tab is described in details.

11.4.1.1. WCS

There are sub-tabs for each of OGC WCS standard requests: GetCapabilities, DescribeCoverage, GetCoverage, ProcessCoverages.

GetCapabilities

This is the default tab when accessing the WSClient. It lists all coverages available at the specified WCS endpoint. Clicking on the Get Capabilities button will reload the coverages list. One can also search a coverage by typing the first characters of its name in the text box. Clicking on a coverage name will move to DescribeCoverage tab to view its metadata.

_images/wsclient_wcs-getcapabilities-tab-1.png

Figure 11.1 List of coverages shown on the GetCapabilities tab.

If a coverage is geo-referenced, a checkbox will be visible in the Display footprints column, allowing to view the coverage’s geo bounding box (in EPSG:4326) on the globe below.

_images/wsclient_wcs-getcapabilities-tab-2.jpg

Figure 11.2 Selected coverage footprints shown on a globe.

At the bottom the metadata of the OGC WCS service endpoint are shown. These metadata can be changed in the Admin -> OWS Metadata Management tab. Once updated in the admin tab, click on Get Capabilities button to see the new metadata.

_images/wsclient_wcs-getcapabilities-tab-3.png

Figure 11.3 WCS service metadata.

DescribeCoverage

Here the full description of a selected coverage can be seen. One can type the first few characters to search for a coverage id and click on Describe Coverage button to view its OGC WCS metadata.

_images/wsclient_wcs-describecoverage-tab-1.png

Figure 11.4 Showing full description of a coverage.

Once logged in as admin, it’s possible to replace the metadata with one from a valid XML or JSON file.

_images/wsclient_wcs-describecoverage-tab-2.png

Figure 11.5 Updating the metadata of a coverage.

GetCoverage

Downloading coverage data can be done on this tab (or the next one, ProcessCoverages). It’s similiarly possible search for a coverage id in the text box and click on Select Coverage button to view its boundaries. Depending on the coverage dimension, one can do trim or slice subsets on the corresponding axes to select an area of interest. The output format can be selected (provided it supports the output dimension). Finally, clicking on Get Coverage button will download the coverage.

_images/wsclient_wcs-getcoverage-tab-1.jpg

Figure 11.6 Downloading a subset of a coverage, encoded in image/tiff.

In addition, further parameters can be specified as supported by the WCS extensions, e.g. scaling factor, output CRS, subset of ranges (bands), etc.

ProcessCoverages

WCPS queries can be typed in a text box. Once Excute is clicked, the result will be

  • displayed on the output console if it’s a scalar or the query was prefixed with image>> (for 2D png/jpeg) or diagram>> for (1D csv/json);
  • otherwise it will be downloaded.
_images/wsclient_wcs-processcoverages-tab-1.png

Figure 11.7 Query and output areas on the ProcessCoverages tab.

DeleteCoverage

This tab allows to delete a specific coverage from the server. It is only visible when logged in the Admin tab.

_images/wsclient_wcs-deletecoverage-tab-1.png

Figure 11.8 Deleting coverage test_DaysPerMonth.

InsertCoverage

Similarly, this tab is only visible when logged in the Admin tab. To insert a coverage, a URL pointing to a valid coverage definition according to the WCS-T standard needs to be provided. Clicking on Insert Coverage button will invoke the correct WCS-T request on the server.

_images/wsclient_wcs-insertcoverage-tab-1.png

Figure 11.9 Inserting a coverage given a URL pointing to a valid GML document.

11.4.1.2. WMS

This tab contain sub-tabs which are related to the supported OGC WMS requests.

GetCapabilities

This tab lists the available layers on the specified server. To reload the list, click on the Get Capabilities button. Clicking on a layer name will move to DescribeLayer tab to view its description.

_images/wsclient_wms-getcapabilities-tab-1.png

Figure 11.10 List of layers shown on the GetCapabilities tab.

Similar to the WCS GetCapabilities tab, it’s possible to search for layer names, or show their footprints.

_images/wsclient_wms-getcapabilities-tab-2.jpg

Figure 11.11 Selected layer footprints shown on a globe.

DescribeLayer

Here the full description of a selected layer is shown. One can type the first few characters to search for a layer name and click on Describe Layer button to view its OGC WMS metadata.

_images/wsclient_wms-describelayer-tab-1.png

Figure 11.12 Showing full description of a layer.

Depending on layer’s dimension, one can click on show layer button and interact with axes’ sliders to view a layer’s slice on the globe below. Click on the hide layer button to hide the displayed layer on the globe.

_images/wsclient_wms-describelayer-tab-2.jpg

Figure 11.13 Showing/hiding a layer on the map.

Once logged in as admin, managing WMS styles is possible on this tab. To create a style, it is required to input various parameters along with a rasql or WCPS query fragment, which are applied on every GetMap request if the style is active. Afterwards, click on Insert Style to insert a new style or Update Style to update an existing style of the current selected layer. One can also delete an existing style by clicking on the Delete button corresponding to a style name.

_images/wsclient_wms-describelayer-tab-3.png

Figure 11.14 Style management on the DescribeLayer tab.

Finally, once logged in as admin, managing downscaled collection levels of a WMS layer is also possible on this tab. To create a new level, it is required to input level parameter (positive number). Afterwards, click on Insert Level to insert a new downscaled collection level of the current selected layer. One can also delete an existing level by clicking on the Delete button corresponding to a downscaled collection level.

_images/wsclient_wms-describelayer-tab-4.png

Figure 11.15 Downscaled collection level management on the DescribeLayer tab.

11.4.2. NASA WebWorldWind

Simple example to setup a web page with a map from a WMS server using WebWorldWind:

<html>
  <head>
    <script src="https://files.worldwind.arc.nasa.gov/artifactory/web/0.9.0/worldwind.min.js"></script>
    <script>
      document.addEventListener("DOMContentLoaded", function(event) {
        WorldWind.Logger.setLoggingLevel(WorldWind.Logger.LEVEL_WARNING);
        var wwd = new WorldWind.WorldWindow("canvasOne");
        var layers = [{
          layer: new WorldWind.BingRoadsLayer(null),
          enabled: true
        }, {
          layer: new WorldWind.CoordinatesDisplayLayer(wwd),
          enabled: true
        }, {
          layer: new WorldWind.ViewControlsLayer(wwd),
          enabled: true
        }];

        for (var l = 0; l < layers.length; l++) {
          wwd.addLayer(layers[l].layer);
        }

        var layerNamesToRequest = ["AvgTemperatureColorScaled"];
        var config = {
          title: "AvgTemperatureColorScaled", version: "1.3.0",
          service: "http://ows.rasdaman.org/rasdaman/ows",
          layerNames: layerNamesToRequest,
          // min Lat, max Lat, min Long, max Long of the requesting layer
          sector: new WorldWind.Sector(-90, 90, -180, 180),
          levelZeroDelta: new WorldWind.Location(36, 36),
          numLevels: 15, format: "image/png", styleNames: "", size: 256
        };

        var wmsLayer = new WorldWind.WmsLayer(config);
        wmsLayer.enabled = true;
        wwd.addLayer(wmsLayer);
      });
    </script>
  </head>
  <body>
      <canvas id="canvasOne" style="width: 100%; height: 100%;"> </canvas>
  </body>
</html>

11.4.3. Python / Jupyter Notebook

OWSLib is a Python package that helps with programming clients for OGC services such as WCS, WCPS, or WMS. To install it please follow the official installation instructions. Example usage for WCS follows below.

>>> # Import OWSLib in Python once installed
... from owslib.wcs import WebCoverageService

>>> # Create coverage object
... my_wcs = WebCoverageService('http://ows.rasdaman.org/rasdaman/ows',
...                             version='2.0.1')

>>> # Get list of coverages
... print my_wcs.contents.keys()
['RadianceColor', 'test_irr_cube_2', 'test_mean_summer_airtemp',
 'test_double_1d', 'INSPIRE_EL', 'AverageChlorophyllScaled', 'INSPIRE_OI_RGB',
 'Temperature4D', 'INSPIRE_OI_IR', 'visible_human', 'INSPIRE_WS_LC',
 'meris_lai', 'climate_earth', 'mean_summer_airtemp', 'multiband',
 'ls8_coastal_aerosol', 'NN3_3', 'NN3_2', 'NN3_1', 'NN3_4',
 'AvgTemperatureColorScaled', 'AverageChloroColorScaled', 'lena',
 'Germany_DTM', 'climate_cloud', 'FiLCCoverageBit', 'AverageChloroColor',
 'LandsatMultiBand', 'RadianceColorScaled', 'AvgLandTemp', 'NIR', 'BlueMarbleCov']

>>> # Get geo-bounding boxes and native CRS
... my_wcs.contents['AverageChlorophyllScaled'].boundingboxes
[{'nativeSrs': 'http://ows.rasdaman.org/def/crs-compound?
  1=http://ows.rasdaman.org/def/crs/EPSG/0/4326&
  2=http://ows.rasdaman.org/def/crs/OGC/0/UnixTime',
  'bbox': (-90.0, -180.0, 90.0, 180.0)}]

>>> # Get axis labels
... my_wcs.contents['AverageChlorophyllScaled'].grid.axislabels
['Lat', 'Long', 'unix']

>>> # Get dimension
... my_wcs.contents['AverageChlorophyllScaled'].grid.dimension
3

>>> # Get grid lower and upper bounds
... my_wcs.contents['AverageChlorophyllScaled'].grid.lowlimits
['0', '0', '0']

>>> my_wcs.contents['AverageChlorophyllScaled'].grid.highlimits
['119', '239', '5']

>>> # Get offset vectors for geo axes
... my_wcs.contents['AverageChlorophyllScaled'].grid.offsetvectors
[['-1.5', '0', '0'], ['0', '1.5', '0'], ['0', '0', '1']]

>>> # For coverage with time axis get the date time values
... my_wcs.contents['AverageChlorophyllScaled'].timepositions
[datetime.datetime(2015, 1, 1, 0, 0), datetime.datetime(2015, 2, 1, 0, 0),
 datetime.datetime(2015, 3, 1, 0, 0), datetime.datetime(2015, 4, 1, 0, 0),
 datetime.datetime(2015, 5, 1, 0, 0), datetime.datetime(2015, 7, 1, 0, 0)]

11.4.4. OpenLayers

Simple example to setup a web page with a map from a WMS server using OpenLayers:

<html>
  <head>
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/openlayers/3.8.2/ol.css"></link>
     <script src="https://cdnjs.cloudflare.com/ajax/libs/openlayers/3.8.2/ol.js"></script>
     <script>
      document.addEventListener("DOMContentLoaded", function(event) {
        var layers = [
          new ol.layer.Tile({
            source: new ol.source.TileWMS({
              url: "https://ahocevar.com/geoserver/wms",
              params: {'LAYERS': 'ne:NE1_HR_LC_SR_W_DR'}
            })
          }),
          new ol.layer.Tile({
            source: new ol.source.TileWMS({
              url: "http://ows.rasdaman.org/rasdaman/ows",
              params: {'LAYERS': 'AvgTemperatureColorScaled'}
            })
          })
        ];
        var map = new ol.Map({
          layers: layers,
          target: 'map',
          view: new ol.View({
            center: [7.5, 53.15178], projection : "EPSG:4326", zoom: 6
          })
        });
      });
     </script>
  </head>
  <body>
    <div id="map" style="width: 100%; height: 95vh"> </div>
  </body>
</html>

11.4.5. Leaflet

Simple example to setup a web page with a map from a WMS server using Leaflet:

<html>
  <head>
    <link rel="stylesheet" href="https://unpkg.com/leaflet@1.6.0/dist/leaflet.css"/>
    <script src="https://unpkg.com/leaflet@1.6.0/dist/leaflet.js"></script>
    <script>
      document.addEventListener("DOMContentLoaded", function(event) {
        var map = new L.Map('map', {
          center: new L.LatLng(40, 52),
          zoom: 3, attributionControl: true, zoomControl: true, minZoom: 2
        });
        var wmsLayer = L.tileLayer.wms("http://ows.rasdaman.org/rasdaman/ows", {
          version: '1.3.0', layers: 'AvgTemperatureColorScaled', format: 'image/png'
        });
        map.addLayer(wmsLayer);
      });
    </script>
  </head>
  <body>
    <div id="map" style="width: 100%; height: 100%;"> </div>
  </body>
</html>

11.4.6. QGIS

11.4.7. Command-line tools

It’s straightforward to make individual OGC WCS / WCPS / WMS requests from the terminal. Examples with curl follow.

  • Make a GetCapabilities request:

    curl "http://ows.rasdaman.org/rasdaman/ows\
    ?service=WCS&version=2.0.1&request=GetCapabilities"
    
  • Execute a WCPS query with a ProcessCoverages request:

    curl "http://ows.rasdaman.org/rasdaman/ows" --out test.png --data-urlencode \
    'service=WCS&version=2.0.1&request=ProcessCoverages&query=\
    for c in (mean_summer_airtemp) return encode(c, "png")'
    

11.4.8. Rasql Web Console

The rasql web console is installed by rasdaman in $RMANHOME/share/rasdaman/www/rasql-web-console. It requires petascope to be running in the background in order to execute queries.

Various widgets are available, with the most commonly-used being:

  • image to visualize a 2D image result, e.g. image>>select encode(..., "jpeg") from ...
  • diagram on csv encoded data, e.g. diagram(type=area,width=300)>>select encode(..., "csv") from ...
  • text to visualize a text result, e.g. text>>select dbinfo(...) from ...

Without using a widget the result is downloaded.

_images/rasql-web-console-example.png

Figure 11.16 Example of a 2D image result.