GEMs Review

Overview:
The Geo localization and Mosaicing System or GEMs for short is a precision agriculture multispectral sensor payload that be used on many different UAS platforms. This sensor was designed to capture RGB, NIR, and NDVI imagery in NADIR. With the purchase of the GEMs hardware, you also receive the GEMs software package as well. This software allows you to process the imagery that you just took and automatically receive orthomosaiced RBG, NIR, and NDVI imagery.

Workflow:
To achieve the final product imagery, there are multiple steps that need to be taken. First, you must mount the sensor to the UAS platform. Whether this be a fixed wing or a multicopter, one important thing to note, the positive charge must be connected to the positive connector. Not doing this on a conventional power system could result in the failure of the hardware or a fire. Once you have the sensor hooked up you must insert a SanDisk extreme 32gb usb jump drive. This is how the GEMs stores its data when in flight.

Now you must consider what the Ground Sampling Distance (GSD) is as well as the Pixel Resolution. This is an important aspect of collecting imagery because you don't want your pixel size to be so large that it doesn't even distinguish separate features that you are trying to study. The GEMs has a GSD of 5.1cm at 400 feet or 2.5cm at 200 feet. The pixel resolution is 1.3MP for both RGB and Mono which comes out to be 1280x1024. Something else that you must take into consideration are all the parameters of mission planning. All these different parameters all relate the quality of the data, but also the efficiency of the platform to accomplish the task at hand. Here are some of the parameters of the GEMs:

Image Sensor resolution: 1280 x 960 pixels

Sensor dimensions (active area): 4.8 x 3.6 mm

Pixel Size: 3.75 x 3.75 μm 

Horizontal Field of View: 34.622 degrees

Vertical Field of View: 26.314 degrees

Focal Length: 7.70 mm

After the flight has been flown, the data exports in a certain folder structure with all the different imagery in their own folder as well as the flight data. These are self proclaimed orthomosaic photos. Now the difference between orthomosaic and georeferenced is that orthomosaics use the photos geometry versus georeferenced photos use the GPS coordinates of concurring points between two different images that are overlapped. Orthomosaics are the best since they are geometrically correct and they take topography and elevation into effect.
 
Once you are ready to run the software it is fairly easy. You simply go into the GEMs software and run an NDVI initialization with the images you took. You then can generate mosaics from them while also computing NDVI, using the default color map, and performing fine alignment. GEMs then gives you the option to export to Pix4D for further processing within there software. What this does is pretty much gets your images ready to be processed in the format that Pix4D wants. Another option is to export to powerOFground which is a cloud based image processing platform that also analyzes your images geospatially. Once you are done with all your other software exports you can look at the five images that GEMs exported for you. Those five are RBG Fine, NDVI Mono Fine, Mono Fine, NDVI FC1, and NDVI FC2 (Figure 1).

Figure 1: These are the five exported images that you receive from the GEMs software along with their pixels values.

You can see from the figure above that you receive different values for each image. For instance between the two NDVI FC images, one has a color scheme that shows yellow or orange as healthy vegetation while the other shows healthy vegetation as green. Which makes more sense (Green)? You can also see that the RGB Fine doesn't have values associated with it. That's because this is purely so that we can have high resolution imagery that is better than something that we would get from an ESRI Basemap (Figure 2).

Figure 2: Comparison of the GEMs RGB Fine image versus an ESRI Basemap image. If you had the raw images files for both of these you can see that the quality difference is quite apparent.

Critique:
Overall I like the GEMs hardware. I like that is has the ability to collect all five of those images at the same time which can dramatically cut down on flight time if you needed to go back through and collect more. The parameters that are set on the system allow for respectable photo quality although it
can always be improved. The system is quite simple to set up on the chosen UAS platform and can be used with a fixed wing or a multicopter. This gives the GEMs an advantage because it can be that one sensor that you HAVE to have because of its wide variety of applications. Not only can it produce high quality images but also NDVI's for precision agriculture.

The GEMs software gets some points for me because it is really easy to run. All you do is click a few buttons to run the initialization and select your photos and your off. This makes it practical for the farmer that is just getting into precision agriculture and wants to use this technology without having to go through some intense training like other softwares would make you do. However, even though it is easy to use, the end product is not up to par. If you look at the Figure 1 above you can see that in most of the images there are mosaicking issues. There are streaks of off color lines that run through the images that can totally throw off your values. This makes for a rather poor end product. I will say that the RGB Fine image is definitely better than what we would get from an ESRI Basemap and that in of itself is worth its weight.

To wrap up I would say that the GEMs is a sensor that can be very applicable but you need to know your needs first. What is your goal? What are your standards and how much money do you have? All of these should go into consideration before you purchase this product because depending on your standards you may be disappointed. If your standards are low and you just need a quick representation of what your fields look like, this sensor will do that job. You will get a high quality RGB image as well as a good representation of how healthy your vegetation is. If that is your need, then the GEMs is your want.