Friday, February 12, 2010

When a foot is not a foot

When integrating FME input/output with Smallworld there is one unit of measure that you need to be particularly wary about... the "foot". Depending on whom you are talking to a foot can be different things.

Consider this...

When people talk about FME calls it Smallworld calls it
International Foot IFOOT :feet
US Survey Foot FOOT :usfeet

This is important in two places when setting up your FME mapping files:
  1. Finding the correct FME coordinate system for the non-Smallworld end of the mapping file
    • If a customer tells you that they are giving you data to import into Smallworld and its units is "feet" make sure that you clarify whether it is in International Feet or US Survey Feet.
    • Looking at the FME coordinate system browser, I notice that many of the State Plane Coordinate Systems use the US Survey Foot.
    • You will need to clarify the same question when a customer asks you to export data from Smallworld. Typically, though, they will ask you for the data in a particular coordinate system and FME will know what units comprise that coordinate system.
  2. Finding the correct FME coordinate system to match your current Smallworld application coordinate system
    • Probably the biggest confusion of feet/usfeet/internationalFeet comes when you are trying to configure an FME coordinate system to match your Smallworld application coordinate system. You need to be absolutely sure that you are using the same "feet" unit in the FME version of the Smallworld application coordinate system.
    • Most application coordinate systems use "cm" or "mm" for units so you won't run into this confusion. Even in US installations where units are measured in feet and inches, the application coordinate system and database coordinate systems are often set to metric units. (Remember that what a user sees on the GIS can be set to show Imperial units even if the underlying coordinate systems use metric units).
A classic sign that you are dealing with the Foot/USFoot issue is if you try loading in data from an external source and it looks like it "almost but not quite" lines up with your existing data. In that case you should carefully review all the coordinate system projections in your data flow process to ensure that the correct "feet" units are used throughout.

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