I want to pass along a tip about confusing field names used in the Ads Factory component for Joomla for geographic data.  I encountered this while customising this component for a client. At first I thought it was a bug, but now I think it’s just an odd naming convention.

Ads Factory, by Romanian developers The Factory,  is a commercial component for Joomla 1.5 which lets you add classified ads to your Joomla site. (My client had me working with version 1.x on Joomla 1.5, but I see there is also a version 2.1 of Ads Factory which is Joomla 1.6 native.) There are quite a few places where Ads Factory includes geographic information: each user record can record a latitude and longitude for that user; each ad can record a latitude and longitude for the advertised merchandise; and there is way to make a “radius search”, i.e. find all ads within a given distance of a user-specified location.

These latitude and longitude values are stored in database fields with name suffixes “X” and “Y”. The user’s latitude and longitude are stored in fields “GoogleX” and “GoogleY” of the Ads Factory user table. Similarly, but not completely consistently, the ad’s latitude and longitude are stored in fields “MapX” and “MapY” of the Ads Factory ads table. The confusion comes in understanding which field stores the latitude, and which stores the longitude.

Latitude is, of course, the signed number of degrees north of the equator of a point on the earth’s surface. It ranges from +90.0 (the North Pole) to 0.0 (the Equator) to -90.0 (the South Pole). Thus, it’s a vertical coordinate. Longitude is the signed number of degrees east of the 0° meridian (roughly Greenwich, England). It ranges from +180.0 to -180.0. My part of North America is 122-123° west of Greenwich, so we have longitudes of -123.0 to -122.0 or so. It’s a horizontal coordinate. This is a well-established convention in many mapping standards.

Tidy Cartesian mathematicians like me use the convention of (X,Y) coordinates, where X is the horizontal coordinate and Y is the vertical coordinate. This is a well-established convention in geometry and graphics (though there are some exceptions).

My first interpretation of Ads Factory field names like  “GoogleX” and “GoogleY” was to interpret them according to the Cartesian convention: X is horizontal, and so stores longitude, while Y is vertical, and so stores latitude. Thus (MapX, MapY) would be (longitude, latitude), the opposite of what one expects from mapping. Odd. I was surprised to find some parts of the code storing latitude in X (the horizontal coordinate!) and longitude in Y (the vertical!), which was surely a bug. I was horrified when it appeared that every part of this code had the same bug!

Then I understood the convention. Ads Factory’s developer appear to have used the (X, Y) convention to indicate just the order of the coordinates, but not their Cartesian meaning.  (MapX, MapY) means (latitude, longitude), as is conventional in mapping.  X is the vertical coordinate, Y is the horizontal coordinate, in the Ads Factory context. If you remember that X means “first”, not horizontal, and Y means “second”, not vertical, the Ads Factory field names are self-consistent, and the code uses them correctly.

I haven’t seen any Ads Factory documentation which explains this, so I hope this note will help some of you Ads Factory enhancers who are using these fields.

Postscript: what did my client ask me to do with Ads Factory for their site?  Modify the radius search to search around the user’s latitude and longitude, instead of a location the user enters. Also, to sort the keyword and category search results by distance from the user. Quite straightforward to do, though it requires customisations to the Ads Factory code that have to be re-done everytime one upgrades the Ads Factory component.

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