Sunday, September 30, 2007

Academic Cycle: Projection Time!

GIS projects must be due soon in many of our classes as folks from all over campus are coming into the GIS Lab for assistance with projecting their data. Funny how everyone from different departments and different classes are all focused on the same thing. Wonder if this has any relationship to how women's menstrual cycles sync after living close together? Does this mean the various departments are psychically, biologically, or spiritually moving in unison? Ha!

Anyway, by far the most popular question in the lab recently has been confusion about defining and projecting data. Map of Texas showing up in the Pacific? Saw this a few times in the lab yesterday. Always the same cause. An undefined shapefile, most of the time geographically referenced, was projected to some form of Lambert Conformal Conic without defining the projection first!! ArcMap can not transform a projection without defining the initial projection first. Students from many disciplines are having a tough time understanding when and why it is necessary to define their projection before re-projecting their data.

As stated in the ArcGIS 9.2 Help, the projection definition "records the coordinate system information for the specified input dataset or feature class including any associated projection parameters, datum and spheroid. It creates or modifies the feature class's projection parameters."

So, how can you know when your data's projection needs to be defined? Well, first clue is ArcMap will give you a popup letting you know. These are not ads. Read them. Second clue is that your data will not be in the correct position, such as a map of Texas in the middle of the Pacific. However, if all of your data is consistently un-defined, then you will only notice this if you add another layer that is properly defined. Tough part is that if a student adds 10 layers whose projections are undefined, and then adds another layer that is properly defined, it might seem as if the defined layer is the one in the incorrect position. Like a democratic vote, un-defined wins 10 to 1.

Why is this confusion happening more than it used to? My opinion is the loss of ArcMap's assumed geographic projection definition. ArcMap 9.1 (and previous versions) would detect and assume that un-defined shapefiles were geographically referenced (decimal degrees). If the data is indeed geographically referenced, then ArcMap's assumption removes the need to define the file before projecting it. However, 9.2 makes no such assumptions. Undefined data must be defined, whether it is geographically referenced or not. Which is better? I sure do not know, but I do know that this is forcing our students (and faculty) to give a lot more thought to projections and datums than they needed to in the past.

7 comments:

Anonymous said...

Not letting folks get away with being lazy and allowing ArcGIS to use Assumed Geographic (which was NAD27, BTW) is definitely better than the alternative. Nobody wants to think about projections, but more critical mistakes have been made because of Assumed Geographic than anything else in ESRI software. It's bitter medicine, but oh so necessary. ESRI got this one right.

mapz said...

Well said!

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