Rectified Aerial Photo
Rectified Aerial Photo – what the heck is that? Please allow me to try and explain. When you rectify something you are stretching or shrinking the object, or photo in this case, to make the object conform to a plane. In context of a photo we are flattening the photo so all the features are on the same elevation. Imagine taking a photo of the Grand Canyon as you fly over it in an airplane. Objects that are close to you like the mountains are going to appear bigger than objects that are in the valley. Closer objects are bigger and far away objects are smaller. Simple right? Ok, now imagine there are two campsites in your photo with the exact same tent. One tent is by the river (low) and the other tent is 4000 feet higher on top of the plains (high). From the image that you took, the tent that is on the plane will appear larger in the photo because it is closer to the lens of your camera, thus the image is distorted. Removing that distortion is called the rectification process. We force the photo to fit between known measured points. Those points are called ground control points or GPC’s. We are essentially stretching and shrinking the photo so that all the features are relative to one another. Once you have a rectified aerial image you can overlay it on a map and all the points will line up or you can sketch from it to create a map.
I know I kind of glazed over it but this is huge! Having a “to scale” photo is really handy. This is the technique that most of our large scale maps are made from. Listed below are a few examples of how you might use a rectified aerial photo.
A farmer might want to know the size of his field this year. Using the photo he can estimate the square footage of his crop for the year. If he knew the amount of corn in a 10 foot square he could estimate the tonnage for the whole field.
Here is another one: say a group of deer hunters have a 500 acre lease with food plots, a river and 7 deer stands. With the rectified photo they could make a map that shows all the deer stand locations, food plots, property lines and deer travel areas for particular times of day (ie. the morning hunt vs. the evening hunt). Wow, we just made a GIS hunting map. There is even a way to make this map digital and upload it to a hand held GPS unit or a smart phone.
Whether you are wanting to create a DIY GIS map of a deer camp, or you need a rectified aerial photo for an ALTA/NSPS survey to meet the requirements of ‘Table A – Item 16’ we can help. Send me an email. I love to talk about this stuff.