How To Georectify Images Using Global Mapper
Landsat Image
Last night, I was searching for an island to work on. One not too large, but one I could finish in a single evening. I came across the above atoll. It is called Reitoru Atoll and is located in the Tuamotus. It looked like a great candidate.
As I do on all my projects, I searched various sources for more detailed imagery and Google Earth (GE) had it...cloud-free, 1-meter resolution of the atoll. I typically don't work in Google Earth to digitize, preferring Global Mapper (GM) instead. Before I can work off of this imagery, it must be loaded into GM and be georectified to ensure the vectors are created where they belong.
Image Rectification with 4 GCPs
Georectification uses an original image to apply the same lat/lon grid to a new image. Given enough Ground Control Points (GCP) applied to shared geographic features, the new image (GE image) can be easily laid on top of the original image (Landsat image). Upon loading a raster image in GM, you are asked if you would like to rectify the image or load it to view. Since I wanted to digitize from the new image, it was necessary to rectify it.
The actual process consists of five-steps for each GCP. Step-1: Select an outstanding feature identifiable on both images. Step-2: Zoom-in to clearly encompass the feature on both images. Step-3: Selet the feature location on the original image. This becomes the initial entry that will become the 1st GCPs lat/lon. Step-4: Select the same feature location on the new image. This is defined by its pixel location. It completes the entry for the 1st GCP. Step-5: Select "Add To GCP List", then move to the next GCP to georectify.
I use something called the Polynomial with 4+GCPs to georectify images. If an image needs to be tugged or pulled this method allows it. Ultimately, the images lay directly on top of each other - the new image on top of the original image. GM allows me to manipulate the new image to guarantee the best format from which to create my Enhanced Vector Shorelines (EVS).
Georectified Google Earth Image
Once the new image is georectified, I am able to create a very high quality product that I can then share with my readers. And that is how you georectify using GM. Isn't it easy?
Enjoy! And have a Happy Thanksgiving!
Labels: Digital Globe, georectify, Global Mapper, Google Earth, Landsat, map
Seems to me like you could have saved a step: Loading it into the Map Rectifier from MetaCarta seems like it would have been a great way to rectify... unless I'm confused. It seems to me that you do have two images for which you already know the geography, so I'm not sure what was achieved by rectifying here, so maybe I'm wrong.
Posted by
Christopher Schmidt |
11/24/2006 05:54:00 AM
I could have tagged the GE image and included lat/lons. However, in this case I took a screen image without coordinates. What I showed in this post was the end result of my georectifying process. Although the new image could have been geotagged making the georectifying process faster, GM's process allowed me to successfully drape the GE image onto the Landsat image.
Posted by
Mr Minton |
11/24/2006 08:38:00 AM
Very interesting!
I am confused (amused) at how little I understand about mapping. For instance, hills/valleys are compressed and actually greater(in length) than they appear from ortho view aerial photo.
Q: Does this mean that maps inherently compress hills & valleys?
Q: Does this (also) mean that parcels of land on hills/slopes actually more net area/acres (ie. Farmable) land than the land survey (flat projection) would state?
Posted by
stevegyro |
12/17/2010 05:27:00 PM