Adapting the raster file

CorrMap acts on image raster files described in section Basic Concepts | Raster  image files. Although the software is generally able to open any type of raster files, even of considerable size and resolution, there may be occasions when you might experience some difficulties opening particular raster files. To overcome this problem, you need to crop or resize the original raster file using image-editing software. For this purpose, we recommend the IrfanView software, a free shareware product, that you can download from its official web site or from this link:


To run IrfanView you don't need to perform any installation, simply unpack the files contained in the ZIP file you downloaded and run the file I_VIEW32.EXE.

The following paragraphs illustrate the tasks you might need to perform on a raster file in case you face problems while opening it with CorrMap.

How to crop a map portion

The size of some raster maps may be too large and subsequently cannot be opened by CorrMap. In these cases, if the geo-referencing or calibration you want to perform only affects a limited area of the map, as it usually happens for the Grid transformation, all you need to do is to crop the interested portion using IrfanView as follows:


1.Click the Open icon shown below to open the raster map:



Fig. BO.9 – Open the big raster map.


2.Select the rectangle to crop. To do this, it is not necessary to use any command, just select directly with your mouse. Once you have selected the rectangle, you can also re-position the sides more accurately by going over them with your mouse (the cursor become a double arrow):



Fig. BO.10 – Trace the rectangle you want to crop.


3.Type Ctrl+Y on your keyboard and the image will be cropped.


4.Select the File | Save as menu option and save the cropped image with a new name, so you don't overwrite the original raster file. Doing so, you can also choose your preferred format (JPG, TIF, BMP, PNG, etc.) and its quality.


5.Open the new cropped image in CorrMap.

How to reduce the raster size

If you need to treat in CorrMap an entire map, not just a small section, and the raster file is too large, you can reduce the size or the resolution as follows:


1.Click the Open icon and open the original raster file as show in Fig. BO.9 in the previous paragraph.


2.Activate the Resize/Resample option from the Image menu:



Fig. BO.11 The Resize/Resample option of IrfanView Image menu.


This command opens the dialog box of Fig. BO.12. Reduce the image size by changing the values of Width/Height cells from 100 to a lower value, such as 50.


Note: this reduction does not loose any information regarding the calculations performed by CorrMap as these are based solely on numerical values, and are independent of the raster image resolution and size.


Fig. BO.12 How to reduce the size of the raster with IrfanView.


3.Select the File | Save as menu option and save the reduced image with a new name, so you don't overwrite the original raster file.


4.Open the new reduced image in CorrMap.