GIS Day function ideas?
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GIS Day is a perfect day to promote GIS within your organisation. Setup something fun - showcase videos like GeoSpatial Revolution or if your in local government, try to woo the people with the budget strings with a video on the role of GIS in Government 2.0
I think its all about showing the non-GIS people in where you work, the potential on what can be done with GIS.
So perhaps also show some live demos of good looking mapping websites like Greeley, St Lawrence and Mapnificent.
Depending on when your reading this, its not too late to do this now - send a nicely constructed email to 'All Staff' :)
geocache party / treasure hunt, where the treasure is booz and pizza
We host a poster contest where all of our GIS staff (although the contest is open to anyone) each make one poster showcasing a GIS project they did in the past year that added value, solved a problem, etc. within our company. Of course we do live demos of our web apps and use the event as an evangelistic opportunity. The posters are voted on by all of the employees that come to marvel at our coolness and gobble up cake and cookies. Any decent bakery can take a image of the GIS Day logo and put it on a cake for you. The grand prize is a free ride (plane tix, hotel, meals, fun) for the winning poster author and a guest to the ESRI UC in San Diego for 7 days - one sweet prize if you ask me. The event is very well attended and we get pop-ins from VPs and usually our CEO who then really get to see who is doing what and the impact that GIS has on our organization. We put up flyers in our buildings and spam people weeks in advance to advertise.