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11 th International Symposium for GIS and Computer Cartography for Coastal Zones ManagementEquirectangular or Lat/Long) is computationally simple and has less area distortion, but more distortion in shape athigher latitudes. It is more appropriate than Mercator for small-scale and choropleth web maps, but is poor for mapsthat need to show complex shapes—such as coastlines—accurately at high latitudes.Data from disparate sources must be processed to standardize the attribute fields, spatial reference system, andmetadata (Haddad et al., 2011). As of this writing, the most popular tools for this are Esri’s ArcGIS 10, QuantumGIS, and the GDAL/OGR Python library. The latter two are included in the OSGeo4W package of open-sourcegeoprocessing software for the Windows platform.Map Servers and ServicesData are delivered from the data source to clients by an HTTP server such as Apache or Microsoft IIS. Georeferencedmaps are unlike other information delivered over the web because they include spatial relationships that mustbe maintained, so it is easiest to serve them using specialized software that extends a web server, called a map server.Most map servers create services that conform to Open Geospatial Consortium (OGC) standards. The OGC is aninternational community of universities, government agencies, and private companies that creates specifications forgeographic data use through a consensus process. Since its creation in 1994, the goal of the OGC has been to maximizethe interoperability of spatial data distribution systems worldwide (Harrison, 2002).OGC standards-compliant services primarily use a RESTful (REpresentational State Transfer) interface, whichuses a Uniform Resource Locator (URL) containing a set of valid parameters to pass a request from client to server.The server responds to the client request according to the values provided for each parameter. An example Web MapService (WMS) request may appear thus:http://neowms.sci.gsfc.nasa.gov/wms/wms?VERSION=1.3.0&REQUEST=GetMap&LAYERS=MOD_LSTD_CLIM_M&WIDTH=960&HEIGHT=600&FORMAT=image/jpeg&CRS=CRS:84&BBOX=-180,-90,180,90WMS is the most well-developed and well-used OGC web service standard. The standard includes a GetCapabilitiesrequest, which returns metadata about the available maps, and a GetMap request, which returns a map image.The client can request any specific portion of the map by applying different bounding box coordinates in the request.Geospatial data is not inherently visible, so the map server must apply a stylesheet to generate an image that is returnedby the GetMap request. This image is not stored, but is rendered dynamically for each request, making themaps very customizable but slower to render for the client.WMS alone is functionally limited by the volume of data in each requested map image. Even at high bandwidths,large map images may take a long time to render and load, detracting from the user experience. Loading time decreasesif map images are served as a series of smaller (typically 256x256 pixel), edge-matching tiles, with onlythose tiles within the user’s viewing frame sent to the client. The tiling approach allows for greater interactivity,with the user able to click and drag to pan the map and zoom in and out without reloading. The downside is thatonce created, the tiles are static images, so custom data requested by the user must be layered on top of the tile map.There are multiple ways to deliver tiles. Some client-side JavaScript libraries can render tiles on the fly by sendingmultiple WMS requests with different bounding boxes that correspond to each tile’s dimensions, then edgematchthe tiles. A faster and more reliable approach is to pre-render the tiles using a server-side script and store themin a specialized directory structure. The parent directory is named according to a fixed zoom level (z), with eachchild directory representing a column of tiles (x), and each image file within that column a separate tile numbered byvertical order (y). The tile URL thus ends in /tiles/z/x/y.png (OpenStreetMap, 2012).There are multiple specifications for tile request formats and addressing schemes. The Tile Map Service (TMS)standard is used by some map server software, but is problematic because it is not interoperable with the more dominantschema pioneered by Google Maps. In 2010, the OGC approved the Web Map Tile Service (WMTS) standard,which builds upon the Google schema, but provides enhanced support for tile metadata and geographic projectionsother than Web Mercator (Masó et al., 2010). Tiles are served by a specialized map server extension. Tilemill, a newdesktop application designed to style, render, and package tile sets, along with its subscription-based remote tilehosting service, provides the most user-friendly option, although currently limited to web Mercator projection.In addition to raster map images, many coastal web maps need to serve vector data to allow for feature queriesand some spatial analysis. The downside to serving vectors is large data volumes, which hamper performance. TheOGC Web Feature Service (WFS) standard provides a method of serving and querying individual features stored in161