It is common that an Apollo Mapping client who purchases satellite imagery does not have a way to view the data on their computer. And that problem is only amplified when the client is a Mac user as the vast majority of spatial programs are built for PC users. In fact, up till the date I wrote this article (and also purchased my own Mac about 4 months prior), our website page featuring free imagery viewers only contained options for PCs. Well, that will all change with this Geospatial Tip of the Month (GTM) as this edition will focus on a freeware program that can open satellite and aerial imagery!
Broadly speaking, satellite and aerial imagery is typically provided in TIFF format. However, it is a specialized type of a TIFF, often called a GeoTIFF, whereby each pixel (or each colored square that makes up the image) has a geographic coordinate (e.g. a latitude, North or South; and a longitude, East or West). And while both Macs and PCs have built-in picture file viewers, they often cannot open an imagery file which can be more than 1 gigabyte (GB) in size; or if they do, your computer runs slowly while viewing the file. Further, none of these built-in picture file viewers will ‘access’ the underlying geographic coordinates which in many cases can be crucial to the use of the data.
A Freeware GeoTIFF Viewer for Macs – Quantum GIS (QGIS)
After combing the web for the best (free) software to view a GeoTIFF on a Mac, I settled on one of the best freeware spatial programs out there, Quantum GIS. QGIS is open-source code base that revivals the functionality of spatial software that costs thousands of dollars. And while QGIS has a plethora of tools that might make the novice imagery user’s head spin, loading a GeoTIFF and working with it is a pretty easy process. If you take a few minutes to watch the YouTube video included with this GTM, we will get you up and running with QGIS right away. For those of you who want to take your imagery analysis to the next level, feel free to explore QGIS’ detailed documentation to see what you can do with your data.
To run QGIS on your Mac, you will need to install four freeware programs found on the KyngChaos.com website, I suggest you do so in this order:
- GDAL Complete
- From the Python Modules, first NumPy
- Then the Python module, Matplotlib
- And finally QGIS
Sorry, yes I realize installing four programs is a bit of a hassle but it was the easiest solution I could find even with this extra burden! A final quick note on installations, you need to be sure you change your Mac’s security settings to Allow Apps from Anywhere to be installed; if you do not do this, the installation process will be blocked!
Opening a GeoTIFF in QGIS
Not everyone will have time to watch the YouTube video included in this GTM, so for those folks, here are the steps you need to take in QGIS to load a GeoTIFF:
- Click on the second icon down in the vertical list of QGIS icons to the left; it looks like a blue and black checker board with a plus in the bottom right corner. See the JPEG here as well as I circled the icon to push.
- Navigate to the location of your satellite or aerial imagery GeoTIFF file, click on it and press Open.
That’s it, you are all set to go now! You can zoom in and out with your mouse scroll wheel, with your two-finger Mac touchpad gestures or with the plus and minus magnifying glasses in the top bar of QGIS.
Color Balancing a GeoTIFF in QGIS
In some cases, you might have received a 16-bit satellite or aerial imagery file that you will need to color balance on your own. While it is not the intent to cover bit depth nor color balancing here, simply stated color balancing is the process of converting a raw imagery file into one with realistic colors. We have covered bit depth in more detail here and then color balancing here. In the short video included with this GTM, we will also show you how to get started using QGIS to color balance your GeoTIFF data.
As a final note to my kindred PC users (I might have a Mac but I run it 99.99% of the time in Windows 7), QGIS would also work exactly like described here when run on a Windows-based machine. A big difference would be the ease of install on a PC as only one file is required!
A five minute or so YouTube video walking you through the process of opening a GeoTIFF in QGIS as well as a few tips for color balancing a 16-bit satellite or aerial image.
Do you have an idea for a future GTM? If so, let me know by email at firstname.lastname@example.org.
Brock Adam McCarty