Previously, I posted a step by step procedure on georeferencing images to align the picture of a road to your existing map data. Sometimes though, you cannot just reference one image but many which can be very disorganized. While it’s okay to just load all the images on mapping software, maybe you want to first simplify the image by means of combining the tiles together. This function also has non-mapping applications, especially when you are patching different tiles of a certain picture.
Fortunately, there is an open source software that will allow you to do this. It’s GIMP, the alternative to Photo Shop, as many people would say online. For my mapping purposes, it was an app worth installing. 🙂 You can download and learn more about this from GIMP’s website. I found much use for GIMP in my mapping practice, because once you export the data into an image or if you are preparing an image to be georeferenced, basic image editing has proven to be very useful.
The first step is to create a new image:
Within this new image, you need to open the image tiles that you want to piece together as layers using the File>Open as Layers button:
After this, you will see the tiles together on the canvas:
On the Layers toolbar, you will see the order of visibility of the tiles. You can then find the Move and Rotate buttons in the Toolbox to move and rotate your tiles as necessary:
Check first if the tiles are aligned in the way that you want. Once you are satisfied with the positions, you can click the anchor button on the Layers one by one to “lock” the position of the tile.
To remove excessive spaces in your composite image, just use the Fit Canvas to Layers command in Image menu (or access it by right clicking on the canvas):
Note that when you save this GIMP file, it will be in .xcf format. If you want just the image, you need to Export the file using the picture extension of your choice. Some popular examples are jpg, png, and bmp.
Once you export, you will also fill out additional settings on the succeeding popup window. Here’s a sample for PNG format:
This procedure is fairly simple, although my younger sister who has had more time to play with GIMP tells me that there are other options or image editing magic tricks from this free app. For the purposes of mapping, I believe this is one of the more useful procedures especially when you have a bunch of images for referencing that need to be connected with one another before the georeferencing activity. I just borrowed a World Atlas image for the purpose of illustrating the procedure but in real life, you can use it to piece together images where you can get attributes like road data or satellite images of an area.