The Common Alerting Protocol (CAP) requires that the targeted alert area is defined. The CAP areaDesc (i.e. the area Description) is a mandatory element. Whereas the polygon element in CAP is optional. However, the use of polygons are becoming quite popular, especially with web-based rendering that uses maps to display the alerting area. The Sahana Alerting and Messaging Broker (SAMBRO) encourages defining a polygon of the alert area which, visually, enhances the perception of the situation. Floods don’t limit themselves to a District, to indicate the inundation area by a District name. Where as a polygon clearly defines the actual impacting area.
Geocodes bring efficiency to CAP messaging; especially, in the archipelagos. It is difficult to draw polygons to include and exclude islands in archipelagos. The CAP on a Map project realized the real need through the users in Philippines and Maldives.The map shows a combination of using geocodes that are used to draw polygons to demarcate effecting areas (gray shaded polygons). While the circle (red) is a freehand drawing demarcating an effective area. If one was to practice freehand drawing of polygons to demarcate land areas it would be quite laborious. Whereas a geocode that has predefined polygons would automatically draw the polygon.
Not only have we seen the effectiveness of geocodes in the Philippines but also see that the Maldivians would require the same. For example, an Atoll, in Maldives, would comprise a cluster of Islands. Some are hard to see on a map. A CAP area polygon might exclude or include Islands that do not belong to the Atoll Council issuing the alert. Predefined polygons (or shape files) with a geocode makes it easy for one to swiftly demarcate the targeted administrative geographic are.
The Philippines Atmospheric, Geophysical, and Astronomical Service Administration (PAGASA) uses geocodes in predefined alert areas to be applied in impact-based warning. Philippines Institute of Volcanology and Seismology (PVHIVOLCS) has also determined the subduction zones that threaten Philippines with tsunami. PHIVOLCS has defined a set of polygons modeled for various levels of intensity and trajectory.
The Sahana mapping module is capable of rendering geocodes and the associated shape files.The geocodes for Philippines are PSGC (Philippine Standard Geographic Codes) which are maintained according to the administrative levels.
These findings, on the real need for geocodes for complementing impact-based alerting, surfaced during the PAGASA conducted National Training Workshop, 3rd and 4th May 2016. The training was part of the CAP on a Map project aimed at training Stakeholders on the CAP standard and procedures along with the SAMBRO tool for originating and relaying messages.