Introduction:

I began my internship at the office of innovation and technology in Philadelphia tasked with Geo-referencing historic images to the most recent imagery Philadelphia has to offer. In order to ease me into the tougher, later dates, they had me begin in 1995, which is much more similar to the most recent imagery than the 1950’s to the most recent imagery.

If you have Geo-referenced before, then you know it can be quite simple, but obviously depends on how similar the images are. But, you would also know how tedious and monotonous Geo-referencing is. My undergraduate GIS classes delved into Geo-referencing a little bit just to get the student’s feet wet, but because I was not versed in the little nuances Geo-referencing requires, I shot myself in the foot and learned the hard way.

Tip of the Sprint:

In order to effectively Geo-reference I suggest understanding what actually saves your control points and what does not. If you don’t know what control points are, control points are the references you place all over your target image when you attempt to tell the software this is where this specific placement on the target image is on the reference image in order to stitch them together seamlessly.

Some images require little to no control points, while others (in my case) seem to take in the 20 range…and sometimes more. Now imagine not understanding how Geo-referencing tracked your control points and going through an entire package of images (140 in my case) only to realize none of your work actually registered.

Let me be the first to say, you and I will never make that mistake again. To cut to the chase, as far as my current knowledge is concerned, there are two ways to essentially save your control point placements. The first way is to ‘rectify’, which will create a new image with the corrections you made. This option is found in the drop down menu of the Geo-referencing toolbar in ArcMap. However, if you do not wish to create new images, option two may be a better fit. Option two revolves around updating the current image through the function ‘Update Geo-referencing’ within the Geo-referencing toolbar in ArcMap. I should have done a better job of understanding this crucial aspect before spending two days ‘completing’ a large task.

Josh