Your SD Card Affects Your Maps

‘A chain is only as strong as its weakest link’

…and when it comes to aerial mapping with drones, all links will be tested.

At DroneDeploy, we define a successful map to have 99% coverage or above. This means that 99% of the intended area to map is stitched together and available to you in the final result.

99% may seem ambitious but it is absolutely necessary.

After noticing some of our customers were not consistently achieving the 99%+ figure, we sprang into action to see if there were any obvious fixes — it turns out, there were.


Your SD card might be holding you back.

Our first tip in our post on How to Improve the Map Quality From Your Drone actually mentions several times that higher overlap and more images equate to better maps.

That means that taking as many images as possible, as fast as possible, will help increase your overall map quality across the board.

A fast SD card allows your drone’s imaging sensor to write the image to its memory faster, allowing it to take the next image quicker — so on and so forth.

Simply put, a faster SD card will allow your drone’s camera to take more images, which means better aerial maps.

Naturally, we had to put this to the test.


The test

For our test, we used a Sony QX1 RGB imaging sensor. Shutter speed was set at 1/1000 and ISO to AUTO. We ran through five different SD cards to test which was fastest. We also tested with no SD card, which serves as a benchmark for this particular sensor.

Sony QX1

The Sony QX1 was configured to take a series of continuous images as fast as it could and results were logged on our machines.

The SD cards vary in price and speed but as a general rule of thumb, the Kingston class 4 card should perform slower than all of the other relatively higher-end cards:

  • Kingston class 4
  • Transcend class 10
  • Lexar 633x class 10
  • Samsung Pro
  • Sandisk Extreme Plus

May the fastest SD card win

The competition

The graph below depicts the results of the cards. Pay close attention to the left group of bars (Mean capture time).

As predicted, the Kingston class 4 was the slowest of the pack. Unfortunately, a decent amount of DroneDeploy’s non-DJI drone mappers were using this card as their default at the time.

This was an incredibly useful discovery and confirmed our assumption that the SD card was, in fact (and quite obviously), a leading cause in hindering an imaging sensor’s ability to capture images at a faster rate.

Lower is better

The Kingston class 4 SD card is spending approximately 1.7s writing to disk vs ~.25s for the higher-end cards.

When capturing and downloading, the bad SD card added a total of ~3s vs. ~.6s for the good cards.

We can infer from this data that a higher-end SD card can capture approximately 10 more images per minute than the lower-end models.

Over a 20 minute flight, this equates to ~200 more images.


The winner(s)

As of this writing, if we had to recommend a SD card to use when mapping with drones, we'd recommend the Samsung Pro for its favorable price/performance ratio (or the SanDisk Extreme Plus micro SD card which is a bit more expensive).

At the time of this writing, the Samsung Pro weighs in at only $6 more than the Kingston class 4 SD card of equal capacity (which was the slowest of the pack).

This is an absolute no-brainer and a very small price to pay for more consistent aerial map quality.

The results

After verifying the drawbacks of slow SD cards, we immediately advised the affected DroneDeploy mappers of the correlation between SD card speed and overall map quality.

Many mappers made the simple switch to faster SD cards. The results below are the average statistics across all DroneDeploy maps.

Instead of reading the graph below from left-to-right, read the below graph from right-to-left.

Weeks 7–5 (in the past) show when the majority of affected mappers were still using the Kingston class 4 SD card.

Weeks 4–0 (to the present) show when SD cards began to be replaced with faster variants.

Week 0 (today) shows increases across the board for average frontlap, coverage, and % of maps above 99% coverage (our success metric).

The numbers

Average frontlap up by 91.2%

(34%→ 65%)

Average coverage up by 8.8%

(91% → 99%)

Percent of maps above 99% coverage up by 42.6%

(61% → 87%)

Caveats

Of course, there are limits to how much of an improvement the SD card’s speed can affect your imaging sensor’s mean capture time. You can also only increase the speed of the SD card by so much until the actual sensor itself is the limiting factor.

For the Sony QX1 sensor we used in the above test, the “No SD card” timing shows that the mean capture time is just under 2s, with the higher-end SD cards bumping it up to just over 2s. Not a drastic difference.

»DJI drones

Currently, we’re limited by DJI SDK, to only capture photos every 5 seconds. The result is very fast SD cards will probably not improve quality (yet). But extremely slow SD cards in a DJI drone, could cause the camera to capture even slower than 5 seconds.

Thankfully, if you do put a really slow card in, a warning message will pop up and advise you of this in the DJI Pilot app.

Regardless of this, we do still consistently high quality and high map coverage coming from DJI drones like the Phantom 3 Pro and Inspire 1.

Like this huge 160 acre map from a DJI Inspire 1 made with DroneDeploy:

Wrap-up

Due to its slow speed, the Kingston class 4 SD card was negatively affecting overall map quality.

  1. By increasing SD card speed, you increase the amount of images taken in-flight.
  2. By increasing the amount of images taken in-flight, you increase the image overlap.
  3. By increasing image overlap, you increase the likelihood of getting 99%+ coverage on your drone maps.

In summary; use a fast SD card when making drone maps. You won't regret it.

How fast is too fast?

Make aerial drone maps using DroneDeploy and find out today.

Start your no risk, no credit card, 30-day DroneDeploy trial at http://www.DroneDeploy.com/signup

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DroneDeploy's Blog

DroneDeploy is the leading cloud software platform for commercial drones. Learn more at www.dronedeploy.com

DroneDeploy

Written by

DroneDeploy is the leading cloud software platform for commercial drones.

DroneDeploy's Blog

DroneDeploy is the leading cloud software platform for commercial drones. Learn more at www.dronedeploy.com

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