Georectification of Images - Image to Image Rectification¶
Lab Materials¶
Download this: GeorectificationData.zip
Lab Content¶
Objective¶
The student will georectify an aerial photo of the Oakwood, Georgia area using a National Agriculture Imagery Program (NAIP) aerial photo as the reference image.
Procedure¶
Georectify an Image¶
- Open ArcMap
- Start a new blank Map Document
- Navigate to your data folder for this lab and add P2010.tif to the map. Note the unknown spatial reference warning.
- Move the mouse cursor over the image and note that the coordinates in the bottom right corner are in unknown units (the numbers given are the pixel coordinates for the image).
- Close your map document and start a new blank Map Document
- Navigate to your data folder for this lab and add ortho_1-1_1n_s_ga139_2009_12.tif to the map.
- Move the mouse cursor over the image and note that the coordinates in the bottom right corner are in meters.
- Navigate to your data folder for this lab and add P2010.tif to the map. Note the unknown spatial reference warning.
- Open the georeferencing toolbar.
- Use the view link table option on the georeferencing toolbar to bring up the link table. As you work through georeferencing P2010.tif you can open and close the link table as needed to review your points and estimated error. Close the link table.
- Ensure that the target layer dropdown on the georeferencing toolbar is targeting P2010.tif.
- You will now create point pairs linking the known coordinates from the reference image (the map) to the ungeoreferenced image (the source). Zoom to the ungeoreferenced image (P2010.tif) by right clicking on it in the table of contents and selecting zoom to layer. Find a clearly identifiable sample location such as an intersection or a parking lot island with a sharp right angle. Click on the add control point button the georeferencing toolbar then click on the location in the image.
- Now zoom to the reference image (ortho_1-1_1n_s_ga139_2009_12.tif) by right clicking on it in the table of contents and selecting zoom to layer. Zoom in on the image to the sample location. Once you have identified the matching sample location click the add control point button on the georeferencing toolbar then click on the sample location in the image.
- Open your link table and see that your point has been added to the table. You have created one point pair. Continue adding points until you have 10 point pairs. The image being georeferenced (P2010.tif) will gradually move in to the correct spatial location during this process.
- Once you have completed your 10 point pairs open the link table. Make note of the total RMS error calculated by ArcMap and the transformation method (it should be set to first order polynomial (Affine)). Click on the save as option (the hard disk icon) to save your points as a text file. The exported text file will contain the X and Y source coordinates along with the X and Y map coordinates.
- Go to the Georeferencing dropdown on the Georeferencing toolbar and select the Update Georeferencing option to apply the georeferencing to P2010.tif. You could also select the Rectify option to create a new georeferenced dataset without altering the original ungeoreferenced image.
Perform a Positional Accuracy Assessment¶
Close your map document and start a new blank Map Document
Navigate to your data folder for this lab and add the newly georectified P2010.tif to the map. Note that you no longer get the unknown spatial reference warning. Move the mouse cursor over the image and note that the coordinates in the bottom right corner are in meters.
Navigate to your data folder for this lab and add SurveyPoints.shp to the map. Review the attribute table. This shapefile contains ground survey points that we will use as reference data in our positional accuracy assessment. Your data folder for this lab also contains a folder, SurveyPointImages, that contains photos of each survey point location.
Create a new spreadsheet in your favorite spreadsheet software. Create the following columns: SurveyPoint, MapX, MapY, ReferenceX, ReferenceY
Zoom to each survey point in SurveyPoints.shp. Hover over the survey point with your cursor and record the coordinates in a new row in your spreadsheet. The SurveyPoint column should contain the current point number and your coordinates should be entered in the ReferenceX and ReferenceY column.
Using the survey point images in your data folder as a reference, determine the location of the survey point in the map image. Hover your cursor over that location and record the coordinates in the MapX and MapY column of your spreadsheet. Repeat this and the previous step of the lab for all survey points.
Once you have collected your data for all of the survey points we can calculate the Root Mean Square Error (RMSE) for your newly georectified image. Create two new columns in your spreadsheet: ErrorX and ErrorY
In the ErrorX column calculate the error for each survey point in the X dimension (subtract the ReferenceX coordinate from the MapX coordinate).
In the ErrorY column calculate the error for each survey point in the X dimension (subtract the ReferenceY coordinate from the MapY coordinate).
Create two new columns in your spreadsheet: ErrorXSquared and ErrorYSquared. For each survey point calculate these columns by squaring the corresponding ErrorX and ErrorY columns.
Create a new cell in your spreadsheet that calculates the sum of the ErrorXSquared column. Create another cell that divides this number by the number of survey points and takes the square root. The resulting number is your value for RMSEx. Repeat this process to determine your RMSEy.
If RMSEx is equal to RMSEy use the following formula to calculate your accuracy:
\[\begin{split}Accuracy &= 1.7308 * RMSEx\end{split}\]Else use this formula:
\[\begin{split}Accuracy &= 2.4477 * .5 * (RMSEx + RMSEy)\end{split}\]
Write a formal lab report on this exercise.
References¶
| [FGDC] | “Geospatial Positioning Accuracy Standards Part 3: National Standard for Spatial Data Accuracy”, 1998 http://www.fgdc.gov/standards/projects/FGDC-standards-projects/accuracy/part3/chapter3 . (accessed September 29, 2013). |