Working with Memory Cards for Drone Sensors
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This tech note discusses best practices for working with memory cards for drone mapping, including what to look for when purchasing a memory card and memory card reader, formatting memory cards, managing memory cards in the field, how and when to transfer images, good file naming conventions, and when you’ll need to geotag images from a telemetry log.
Memory Cards on Drones
Photos taken from your drone are very large and generally not automatically transmitted to the controller or tablet (some mission planning software such as Pix4D Capture provide an option to download images after a mission, but we always disable this because transmitting large files over WiFi takes ages). Rather the images are saved on memory cards on the camera itself. Memory cards are thus a critical part of the data chain, and yet they are often neglected. Consider the following suggestions to help ensure that your memory cards do the job they are expected to do.
Get the right type of card
- The Parrot Sequoia takes a standard SD card. GoPro, most DJI cameras, and many other types of cameras take microSD cards. Read the documentation that comes with your camera to find out which size it takes.
- A typical flight can produce 8-15 Gb of images (depending on the sensor and the area), so get a 32Gb card or larger. Note however there are risks to putting the images from many flights on the same memory card. You can buy 128Gb memory cards that can hold images for as many as 10 flights, but this is putting a lot of eggs into one basket. If something goes wrong, that's a lot of data to loose.
- Don't get cheapo, low-end memory cards. Spend a few more dollars to get a higher end card with faster transfer speeds and more rugged construction. Consult your camera documentation for recommendations, or look for cards marketed for drones or video. A SanDisk Extreme 32GB MicroSDHC UHS-I Card, for example, costs ~$17.
- Have extra cards as backups and so you can use a fresh card as often as possible.
- Come up with a system for storing and labeling your cards. Materials you can use include little cases, sticky notes, ziplock bags, sharpies, etc.
Get a good memory card reader
You need a reliable, fast reader to transfer images from the memory card to your laptop. Some laptop have built-in memory card readers. You can also buy USB memory cards readers. MicroSD memory cards almost always require a reader, and sometimes come with one. If buying a USB memory card reader, look for USB 3.0 reader which is a lot faster when plugged into a 3.0 USB port.
Reformat cards after each mission
After you have downloaded images, reformat the memory card using a 'quick' reformat. You can do this on your computer or the camera, it doesn't matter. The purpose of reformatting the memory card is to get rid of all traces of previous flights, including folders and hidden files created by the camera. Leaving these files behind can (and have) caused problems that can result in data loss. To format a memory card in Windows, right-click on the memory card in Windows Explorer, and select ‘Format…’ from the pop-up menu.
CAUTION: Formatting drives wipes them off completely and usually irrevocably. Be sure to format the correct drive! If you don't feel confident in your ability to determine which drive is your SD card, ask someone for help because the price of making a mistake can be huge.
Format with the correct file system
Most cameras designed for drones are relatively new and can read and write memory cards formatted with any of the common filing systems: FAT32, NTFS, or exFAT.
Some cameras however, including the Parrot Sequoia, can only read memory cards formatted in the FAT32 file system. When you format a memory card with Windows, FAT32 is the default file system for memory cards = 32Gb, but double-check to make sure it is selected (see Figure).
However for large capacity SD cards (>32Gb) Windows won't even give you the option to format it as FAT32. Windows has a rule again creating FAT32 partitions larger than 32Gb (for perfectly good reasons). This limitation applies to all utilities that call upon Windows, however third party utilities exist that will allow you to format a memory card >32Gb as FAT32. If you Google '64GB SD Card FAT32', you'll find various options.
Manage your cards in the field
If you use more than a couple of memory cards in the field, you’d do well to put some thought into how you’ll protect and keep track of them. Your tools include memory card cases, little stickers, Sharpies, little plastic bags, a handwritten flight log, etc. To get the longest life from your memory cards, keep them protected by storing them in cases or bags.
Don’t move cards between cameras
You can use the same memory card on the same camera for 2 or 3 consecutive flights, however don’t move the card between cameras unless you have downloaded the data and reformatted the card. Moving a memory card between cameras could ruin the files already on them.
Eject the card properly
Before removing a memory card from a camera, make sure the camera is turned off (this probably requires turning the drone off). This will prevent data on the card from getting corrupted due to an interrupted write operation.
Before removing a memory card from your computer, use the ‘Safely Remove Hardware’ button in the system tray.
Inspect the images before you leave the field
You are strongly encouraged to copy the images from the card to a laptop, and view a few of them for quality, while still in the field. This could prevent a return visit in the event that images got corrupted, the camera lens wasn’t focused, or other problems.
Use Good File Naming Conventions
Use descriptive names when naming the folders where your image will be saved. For example, the folder name could consist of the following:
date + flight + location + platform + sensor + elevation
Learn How to Geotag Images from the Telemetry Log (if needed)
If you are using a camera that does not have a built-in GPS sensor (like most GoPros), you’ll need to also download the telemetry log from the drone and pair them up in the software by matching the time stamps. Telemetry logs are usually saved in the drone’s internal flash memory (not the camera), and can be retrieved by connecting the drone to the computer through WiFi, Bluetooth, USB cable, or (another) memory card transfer. Make sure you know how to get the telemetry log off the drone before you fly a real mission, and retrieve it at the end of each mission. For details on getting the telemetry log from a 3DR Solo, see Geotagging Photos from a Telemetry Log.