Omololu Babatunde
Lab 4
Part II. Answer these questions in a Word document and save it to the same folder:
1. In your own words, explain the process of georectifying a digital image.
First open up Arcmap and load a basemap and make sure the base map is in imagery form. Then in google earth select a reference point that is in the image that you want to georectify, find the decimal degree coordinates of the point and then enter the coordinates into Arcmap. Once you enter your coordinates in the “Go to XY” button of ArcMap select “add point” so that your point can be seen easily and then click “zoom to”. Once you zoom to the area it is time to add the image you want to georectify to your basemap. Click “add data” and select your image, then in the georeferencing tool bar click “fit to display”. Once you complete those steps you can click your images check box in the table of contents and your image should appear on top of your base map.
Now it is time to properly align your image to the base map. You are going to do this by adding control points. In the georeferencing toolbar click “add control points.” Then proceed to find landmarks (like intersection, or buildings etc...) that both your image and your basemap share and connect them with the “add control points” tool. It is important to find control points all over the image you are georectyfying. After you’ve added a few points click the “view table” button in the georeferencing tool bar to see the residual error of each point. Points having high residual error may need to be erased or re-plotted.
Once you believe that your image aligns well with the base map (I would say one must have at least six points) it is time to rectify your image. First in the georeferencing tool bar click “Transformation” and see which transformation (1st, 2nd, or 3rd order polynomial) makes your image align best with your base map.
After you chose a transformation in the Georeferencing drop down menu click rectify.
2. How might you or another researcher use this technique in the future?
If a researcher was interested in the effect suburban sprawl had on a location they could they could georectify and old image of a space to a new image and analyze how a space has transformed over time. The researcher could look at the number of new suburbs created and the roads and highways built to allow those living in the suburbs to be connected to the rest of the city, or state.
3. Explain the different rectification methods available. Which did you choose?
The different rectification methods available are 1st Order polynomial, which distributes error across an area, 2nd order polynomial, which has a global area distribution, and Adjust which does a local points distribution of error. I found that the 2nd order polynomial made my residuals the smallest and made my image align the best so I chose that one.
4. What happened to the RME as you added more points? Why might this be?
I found that my RME decreased as I added more points. Since the RME measures the accuracy of the points I took this to mean that as I placed more points and my image became more aligned with the basemap the accuracy of my placed points increased.