A Mini But Mighty GNSS Mapping Peripheral

Took a Bad Elf Flex Mini for a test drive. Had a lot of fun with this affordable, capable, and very easy-to-use asset/GIS mapping peripheral.

In keeping with their focus on providing tiered solutions for field mapping and surveying, Bad Elf recently released a nifty younger sibling for their flagship Flex system. Whereas the Flex can be operated as a full surveyor-grade-precision rover (e.g. centimeters), the Mini fills a sweet spot for asset/GIS precision fieldwork (e.g. meter, decimeter, sub-foot). All of the systems in the Flex family come with tiered purchase options, depending on your precision needs.

Mapping Pucks

Let’s step back a bit and look at how the market for peripheral GNSS receivers for asset/GIS mapping evolved, and how Bad Elf systems evolved with it. The legacy of what folks called “GIS mapping” handhelds began with single frequency systems, for instance, receivers in a backpack with an external antenna on a short pole, and often a tangle of wires connecting to proprietary data controllers running now defunct operating systems (I for one, do not pine for that era). Initially, observations had to be post-processed just to yield meter or sub-meter accuracy. There were incremental advances, like dual frequencies and post-processed results in the decimeter, or sub-foot range. Combination receiver/antenna/data controller handhelds became all the rage. Eventually, there were combo mapping handhelds that could achieve centimeters results, but by then the game had changed dramatically.

Beginning about a decade ago, the technology of mobile operating systems, receivers, and GNSS underwent a rapid evolution. The rise of non-proprietary (consumer segment-driven) tablets and phones gave rise to an era of “bring your own device”. Namely, connecting your phone or tablet to an external GNSS receiver/antenna brings high precision to just about any app. Especially popular are professional asset/GIS applications like Esri’s Field Maps; though there are many now to choose from. I used to refer to these peripherals as GNSS “pucks”, and test-drove many as they evolved.

Bad Elf, a manufacturer of GNSS hardware, software, and solutions has been making nifty and affordable GNSS pucks for many years. There were several (very small and very puck-like models, including the Bad Elf Surveyor (as a surveyor, that name made me wince) that could use external corrections sources (SBAS, NTRIP-based RTK, and network RTK). Under the right conditions, this could yield reliable sub-meter precisions for mapping apps on a phone or tablet. Since discontinued, the success of the pucks led Bad Elf to develop the Flex, a full survey-grade-precision capable receiver/antenna that could be used as a GNSS peripheral to phones and tablets. The Flex came with tiered purchase options, and a rather creative pay-as-you-go option, using tokens for precision only when you need it. I test-drove the Flex, and performance-wise, it kept up with the dedicated survey rovers I’ve tested. I found that various end users were using the Flex for asset/GIS mapping applications, and survey grade work—sometimes for both within the same organization.

Then, to fill in the asset/GIS precision niche of the discontinued pucks, the Flex begat the Flex Mini.

The Basics

The Mini is well, mini—only 3.5” (9 cm) tall and weighs only 7 oz (220 g). It is USB rechargeable, connects to your Android or iOS phone/tablet via Bluetooth, and has a small status screen. There is an optional phone/tablet mount, a ¼”x20 to 5/8”x11 adapter for standard field poles, and a suction cup mount for mobile applications.

The tiered purchase options start with the Mini Standard, listed on their online store at $500 USD. The Standard can use regional free satellite-based augmentation (SBAS). For example,  WAAS in North America, EGNOS in Europe, MSAS in Japan, etc. In autonomous mode (no corrections of any kind), either Mini will get around 2.5-3m.  With SBAS (on the Standard), you can get 1.5-2 m results. Suitable for coarse asset/GIS mapping.

For $1,500 USD, you can get the Mini Extreme (or upgrade your Standard for the difference). The enhanced SBAS can bring results to 1 m; without any subscription needed. The Extreme can tap external correction sources, like NTRIP-based RTK and/or network RTK (RTCMv3 format corrections). RTN If you have, for instance, local IP enabled RTK bases, or a state/regional RTN (subscriptions might be required) you can be working in the decimeter and sub-foot precision ranges (but more on that later). There is also an option (listed in the Flex app) to tap the Point One Navigation Polaris corrections service. Point One operates a network of over 1400 fixed bases in the U.S. and parts of Europe. Bad Elf can arrange Point One access.

The Mini supports 4 GNSS constellations with global coverage: GPS, GLONASS, Galileo, and BeiDou.  Plus, two regional constellations: IRNSS/NavIC (India) and QZSS (Japan). However, unlike much more expensive survey-grade rovers (that can support as many as 5 signals from a single constellation like Galileo), the Mini only supports one or two signals from each supported constellation. There is no option to log static files for post-processing (at this time); you’ll be working in autonomous mode, SBAS, real-time corrections, or Point One mode. These factors kept the price low. And while there are performance limits compared to full survey rovers, the Mini was designed as “fit-for-purpose” for asset/GIS mapping.

Test Drive

I always try to use a device without looking at a manual, just to see how intuitive it is. Field gear in general has gotten a lot better over time. However, the Mini was a particularly good “cold turkey” experience. You open the box and find a card with setup instructions (illustrated with the signature Bad Elf cartoon elves). Essentially it took only one action after turning it on; you snap the QR code on the bottom of the device with your phone/tablet, and it prompts you to download the free Flex app (if you do not already have it installed). It connects to the device and walks you through a few initial questions. You would then do the applicable settings in whatever field mapping software you want to port positions to (e.g. Esri Field Maps). Then you are ready to go; nothing more to it.

If you have the Extreme, you can then choose if you will use the Point One, or other external corrections services. Let’s say you will connect to a local or state RTN; there is a standard NTRIP page to put in your credentials, caster/port, and choose mountpoints. The satellite info and predicted precisions are displayed. While users will mostly be using another app for mapping, you can log individual or groups of points with the Flex app; this can be quite handy for testing and troubleshooting. One feature I found particularly great for testing was continuous logging. After initialization (typically 30 seconds+) I let it run in continuous mode for a few minutes set up over a point. Then you can look at the results and averages in the app, and/or export the logs to look at in a spreadsheet. You can even export mini-maps of observations.

The results were what I had expected; they met the published specs. In purely autonomous it was 2-5m, in SBAS (on the Extreme) it averaged around 1m. Connecting to a local RTK base (about 5km away), and a regional RTN, brought much higher precision results. A fix would take about 30 seconds, but longer under light canopy. When the app indicated “fixed” for either, it displayed a horizontal precision of 24.7 cm. I think this is a default value, as the results were more precise. I did the testing on a National Geodetic Survey certified calibration baseline, so I could check relative results, comparing published and observed inverses between various marks. With RTK/RTN I was seeing values between 5-15cm. With Point One (a base was 7 km away) it was mostly between 10-20cm, but with quite a few outliers as much as 30cm. I would recommend doing some continuous sets in different environments so you can get a feel for how it will perform.

It did struggle a bit under moderate and heavy canopy, as well as in high-multipath environments. But anyone who has used a mapping handheld is used to this, and there are instances (even with survey rovers) where you need to move out in the open and do some kind of offset.

Overall, this is a pretty impressive little device. Let’s be clear; this is not a survey-grade rover—but it was never intended to be. If you want that, go take a look at the Flex. For intended uses, for the price, with the flexibility it affords, and considering the sheer simplicity of operation, I think that Bad Elf has found a worthy (and much improved) successor to its mapping pucks.