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Comparing GPS Traces of 3 Readilly Available Devices

Posted by bobwz on 5 November 2020 in English (English). Last updated on 6 November 2020.

This will be my third and final installment in a series of articles discussing device I’ve used when preforming surveys when contributing to OSM.

In the first article, I talked about accuracy limitations associated with GPS signals while the second gave an overview of four specific mapping devices I’ve personally used. In this article I’ll compare the accuracy of three devices previously discussed using GPS traces.

Test Setup

To test device precision, I recorded GPS traces along a hike with a variety of line-of-sight obstructions to the sky with each device recording at the same time. The traces were assigned colors to help differentiate each device:

  • Samsung Galaxy S10e - Orange
  • EcoDroidGPS - Green
  • Garmin GPSMAP 66sr - Purple

The arrangement of the devices are as follows:

  • The EcoDroidGPS was placed in a backpack with the reciever on an antenna mast above my head.
  • The Garmin was attached to one of the backpack straps with the built-in antenna stub poking just above my shoulder.
  • The S10e was in one of the side pockets of the backpack.

The EcoDroidGPS and S10e location information were sampled every one second within a GPX track using the app OsmAnd. The 66sr records as an “activity” with location information sampled every one second. The resulting GPX track was downloaded from the device.

Results

The Bridge

Before arriving to the trailhead, I had to walk along the shoulder of a bridge spanning a medium size river. The bridge is long enough to offer a best case scenario for accuracy. There is a good 45 degrees of sky visible from most angles in the center of this bridge.

The result of a stretch of road with minimal interference is quite predictable, all three traces are stacked right on top of each other without significant deviation.

GPS trace over bridge

The Gravel Path

The trail begins as a wide gravel path visible from satellite imagery.

In this section of the trail, I stayed to the far right which is more accurately portrayed by the green EcoDroidGPS and purple Garmin traces. It’s interesting to note that the orange S10e trace follows the curve of the others, but is offset about 4 meters to the left before rejoining the green and purple tracks towards the top of the picture.

GPS trace along wide path

The Wetland Trail

At this point, I took one of the side trails which starts in a mild wetland area before breaking to thick woods. In fact, from the satellite imagery, the trail is not visible.

Looking at the traces, the green EcoDroidGPS and purple Garmin traces agree with each other, especially during the first part of the trail where the tree cover is not as dense. At this point, the orange S10e is surprisingly more precise than the previous gravel portion, but still lazily tracks the actual path.

GPS trace in woods, part 1

The Out-and-Back

This section of the trail is an out-and-back, so we actually see two pairs of each trace as I return along a path I’ve already taken. The path tread here is maybe half a meter wide which means I should be walking the same path both ways.

Here we see both the greatest deviation between devices and location, despite walking the same path in a different direction.The purple Garmin trace is the most consistent while the green EcoDroidGPS brackets it on each side. The orange S10e trace preforms the worst in this situation.

GPS trace in woods, part 2

Conclusion

Which device should be used when surveying for OSM contributions? I think the answer depends on your situation and what you’re interested in mapping. A smartphone struggles with mapping smaller ways, but is sufficient when you can include satellite imagery, existing data, or even using multiple GPS traces and averaging the results. For greater accuracy, if you don’t mind the antenna mast sticking out of a backpack, the EcoDroidGPS is a phenomenal value. Since it uses a USB receiver, it can be upgraded as better receivers come to market. The Garmin is a perfect example where spending more money on a single GPS technology does not necessarily equate to better accuracy. It is certainly better than the smartphone, but not much is gained over the much cheaper EcoDroidGPS.

For me personally, I’ll be transitioning to using the Garmin, mostly because I already own it. I tend to view it as an extremely robust outdoor navigator whose data can later be used in OSM contributions. It isn’t as fragile as a phone or the EcoDroidGPS outdoors and I expect the accuracy to increase over time as features are added to the device. If it does, I’d like to post about that as well.

For everyone else, if you aren’t in the market for a more precise device to use in your surveys, wait for a smartphone upgrade. Multi-band and multi-GNSS devices will become common and will represent the next appreciable increase in precision, something that will simply be included in new phone models going forward. If a smartphone isn’t your thing, the effect of including them in a smartphone should help increase the availability of this technology and lower the cost from other vendors.

My Experiance With Devices Used in OSM Surveys

Posted by bobwz on 4 November 2020 in English (English).

In this article, I’ll continue sharing my experience on finding a generally-available commercial device to be used with preforming surveys for contribution to OSM. I previously wrote about GPS precision and why, by itself, it is limited to meter precision.

Below are the devices I’ll be discussing based on my own usage. I’ll share how I used them and my thoughts on each. In a final article of this series, I’ll provide a GPS trace comparison of the EcoDroidGPS, smartphone, and Garmin GPSMAP 66sr as well as which device I’d pick if doing it all over again.

Modern smartphone (Samsung S10e)

The first device I used for surveys and general contributions to OSM was with a smartphone. I currently use a Samsung S10e with the below Android apps depending on the complexity of my contribution:

  • OsmAnd - Used for adding basic POIs and creating GPS tracks with built-in plugins.
  • StreetComplete - Used for casual contributions and tag refinement on existing features.
  • Vespucci - Used for more complex editing of most features in OSM. Offers an editing experience like iD or JOSM, except on mobile.

Not surprisingly, since I generally have my phone with me, this is the most convenient when I stumble across mapping opportunities in the wild. The location accuracy is acceptable. It can be used to map larger roads and general POIs without issue, but is not accurate enough to trace a sidewalk or trail. I’ve also noticed that accuracy improves in urban areas likely due to the ability for Google’s location services to supplement GPS data with nearby WiFi access points and Bluetooth.

Using the same phone for both general daily tasks and mapping places a heavy load on the battery. I’ve been out on plenty of “spur of the moment” surveys that had to be cut short or rushed due to a dimishing battery charge. When mapping GPS traces, I usually use an aggressive logging setting of 1 second combined with higher-than-normal screen-on time as I record POI information. As such, if I’m mapping an area for more than a couple of hours, it’s imperative I bring a battery pack or have access to a charger. The battery hinderance tends to crop up more often than expected, but can be addressed by planning ahead.

With recent announcements of new phones incorporating multi-band GNSS chips, I expect we’ll see smartphone accuracy increasing as multi-band GNSS chips become more prevalent.

EcoDroidGPS

My primary device for detail mapping is the EcoDroidGPS. It is intended to be used in vehicle, but can be adapted to provide excellent precision inexpensively. The EcoDroidGPS includes the following:

  • USB GNSS receiver
  • Raspberry Pi Zero W
  • Car charger

EcoDroidGPS

While mapping, I’ll power the unit using a power bank in my bag and a makeshift antenna mast with the USB receiver on top of it connected to my phone via Bluetooth. The receiver sits above my head which really helps to remove interference.

Since this EcoDroidGPS works by feeding location information to your phone via Bluetooth, an additional app is needed to bridge between your phone and the EcoDroidGPS as a mock location provider. I use the app Bluetooth GNSS made by the same company as the EcoDroidGPS, but any app providing mock location data via Bluetooth should work. Once set up, I’ll use the apps listed in the smartphone section to do my mapping.

Regarding accuracy, the EcoDroidGPS is precise enough to map trails, sidewalks, and residential roads confidentially. If I’m intentionally visiting an area to preform a survey, I’ll plan to use this device. The entire arrangement is cumbersome, but I’m happy with the precision this device offers.

Garmin GPSMAP 66sr

Garmin GPSMAP 66sr

The 66sr from Garmin is the company’s newest (launched fall of 2020) outdoor handheld. Based on the footprint of the existing GPSMAP 66 series, the 66sr was introduced with multi-GNSS and multi-band features promising greater precision over existing navigation products.

As someone who does not upgrade their smartphone each year, the announcement of the 66sr was extremely interesting to me as a solution for mapping trails, sidewalks, and less established ways using more accurate multi-GNSS receivers. I viewed the dedicated navigation unit as a solution the battery woes of a smartphone and the cumbersome set-up of the EcoDroidGPS.

I don’t think this unit is for everyone, especially if the primary use would be a survey tool for OSM contributions. It’s expensive, the interface can be confusing, it’s difficult to understand how the myriad of apps and management programs fit together, and it doesn’t offer many unique features that cannot be found in a recent smartphone. However, if you find yourself in extended outdoor activities with a potential for spotty cell coverage, the perspective changes to a rugged outdoor navigation device that can also used for OSM contributions.

The 66sr’s multi-GNSS receiver results in greater accuracy compared to a smartphone, but ties the accuracy of the EcoDroidGPS. I can accurately map narrow trails and sidewalks. The 66sr seems to have been launched a bit pre-maturely in regards to accuracy-related features. The product page and owner’s manual highlights the inclusion of RINEX logging, WAAS/EGNOS, and multi-band configuration options, all of which are currently missing in the 2.80 version of the device. In contacting support, they promise many of these features are still in development. WAAS/EGNOS and multi-band settings can increase accuracy while RINEX can be used in post-processing to gain centimeter accuracy and confirm all reported accuracy systems are working as promised.

DIY (simpleRTK2B or Arduino solutions)

The final group of devices are ones that require significantly more elbow grease and know-how to get up and running, but can have potentially greater results than off-the-shelf solutions previously mentioned. Every aspect of these can be configured, since they must be assembled and coded your self.

As mentioned in the previous article“Accuracy and Precision in GPS Units”, there are accuracy limitations when using a single device and most survey-grade two-device set-ups with a base station and a rover costs thousands of dollars. The simpleRTK kits from ardusimple may be a solution to lower the cost involved. I have not used these kits, but everything needed to get a base station and rover combination up and running can be purchased and researched on their site.

Returning to single device location devices, Arduino components can also be purchased and a program written to log GPS traces or way points that can be processed later. This has the advantage of adding and removing features important to you and developing an enclosure to fit. Although I never successfully completed the project, below is a lit of hardware and resources I used:

Next Up

This series is ending up to be much more extensive than I initially anticipated. Since this article is getting a bit long in the tooth, I’ll save my GPS trace comparison for the final article and share my final thoughts.

I hope this device comparison has been helpful to folks who may be interested in exploring other options when preforming surveys for OSM contributions.

Accuracy and Precision in GPS Units

Posted by bobwz on 2 November 2020 in English (English).

I’ve spent a fair amount of time looking for the “perfect” consumer device to be used in my on-the-ground surveys that provides better-than-average accuracy without spending thousands on survey-grade equipment. This article series is an attempt to catalogue my experience to those who are interested.

Before diving in, I’d like to review the obstacles to accuracy when surveying using GPS-enabled devices. While this subject may be common knowledge to those familiar, it took me a considerable amount of time to understand some of these points and was a major driving factor in my quest for the “perfect” device.

GPS vs. GNSS

Throughout this article I’ll use the term “GPS” to refer to the systems and satellites that provide longitude and latitude location information to a user. Please note that this is not 100% accurate, however, since “GPS” is a specific satellite (known as a “constellation”) location system among many. In fact, GPS is part of a general Global Navigation Satellite System or GNSS. Other GNSS systems are as follows:

  • GPS
  • GLONASS
  • Galileo
  • BeiDou

I’ll be using “GPS” in an attempt to minimize confusion to those (like myself) who were unfamiliar with this distinction until now.

Precision Limitations

In brief, GPS works by analyzing a time code that is continuously sent by satellites in orbit around Earth. Receivers take that time code from multiple satellites and determine how long it took for the receiver to receive the time code data. This information is then used to calculate a longitude and latitude location on Earth.

Since GPS location is based on calculating the time it takes to beam a signal from space, anything between the GPS antenna and the satellites will impact the time it takes for the signal to arrive and affect accuracy. Buildings, trees, cars, even the atmosphere impacts this timing. This can be seen by logging your GPS position with a smartphone in a field; despite standing still, the position recorded will vary over time giving an idea of precision.

What all this means, is that a GPS signal by itself has a maximum precision. For GPS specifically, the maximum precision is around 5 meters for civilian use. If standing in a field using GPS signals alone to determine your location, you can expect the reported location to be within about 5 meters of your actual physical location on Earth. Add in a small antenna like which is typically found in smartphones and line-of-sight obstructions like buildings, trees, or vehicles, and it is not unreasonable to see the precision degrade to 10 meters or more.

Precision Improvements

To breakthrough this maximum precision limit, receivers must incorporate additional data sources to resolve their location. Most constellations can additionally be augmented by using satellites or ground-based reference stations that feed correction data to receivers. In the case of GPS, these augmentations can increase precision from 5 meters to 2-1.5 meters. Modern smartphones can use cell towers and nearby WiFi hot-spots to improve location data.

Another option is to use two devices. One is positioned at a set location and does not move. The second device (known as a rover) moves about the survey location gathering points of interest. The device at the set location receives GPS information and compares it to it’s set, known location to calculate GPS errors received in the immediate area. With this information, the device can send corrections to the rover gaining precision at the centimeter level. This process, known as real-time kinematic (RTK), is commonly used in professional surveying.

Recently, antenna and receivers have begun popping up that use multiple GPS constellations to refine their precision. Unlike the vast majority of receivers that use one constellation at a time (for example, GPS or GLONASS), these “Multi-band GNSS” receivers will use the data from multiple constellations at a time to determine location. This increases the number of satellites available to calculate the position of the receiver and the number of data points available to help mitigate atmospheric or local interference. I expect these will grow more common over time.

So What?

There are many more topics discussing how GPS does what it does and additional technologies involved, but I wanted to start at this more basic level. It took me a while to understand that GPS accuracy and precision is essentially fixed in the consumer market. I was spending a lot of time shopping around for GPS units without understanding the mechanism in which they function. I thought that buying a more expensive unit or “this years model” I would get better results until gaining centimeter precision. I hope this information reveals that accuracy improvements come from how a device resolves its position, rather than what year it was made, how it looks, or just slapping a “high precision” sticker on the box.

In the next article, I’ll finally get to the devices I mentioned at the beginning and walking through my experience with them.