There are a lot of mobile GIS apps on the market today. Even back in 2012 when we first switched completely to mobile workflows, there were a lot of options in the iOS space. But the problem has always been that mobile GIS app-makers seem to focus on the data sheet form collection and cloud components and then leave the GNSS interface and actual location data collection as an afterthought. While the cloud team aspect of mobile GIS and complex data form capability are what makes mobile so great, the GNSS component is equally important. This disconnect between the GNSS component and data collection software in mobile solutions has been difficult and frustrating for folks that have been accustomed to having access to satellite metadata and the ability to average point collection with software like ArcPad or vendor specific data collection software on handheld GNSS units.
So, the problem for people that need to record satellite metadata, like estimated accuracy, with their point collection is that there hasn’t been many (any?) mobile GIS apps with that capability. Matters have been even worse for RTK users because the corrections from the RTK network are in datum NAD83 and the map and data collection for mobile GIS apps (including Collector) are in WGS84 (ITRF08). This causes the user to see their location cursor as being shifted 4 or 5 feet on the map image compared to where they are actually standing in the environment. Not surprising considering the GNSS receiver is providing location data in one datum and the app’s map is in a different datum.
To demonstrate this, I connected an iPad running Collector to an Arrow 200 RTK receiver connected to a local RTK network. Then I placed the receiver directly adjacent to the south side of a large mailbox that can be seen in the aerial imagery (to the right). The Arrow 200 was receiving real-time centimeter corrections in NAD83, so with Collector set to Default (WGS84), it appears on the map that I am standing 3.7 feet away from this white mailbox when I am actually located right next to the south side of it. This is a good visual of the problems RTK users experience with mobile GIS apps.
But there will soon be a fix for this problem. Summer 2016 brings us Esri’s next update that makes Collector a sought-after data collection app for high-precision GNSS and RTK users. This update will finally make it possible for mobile GIS users to collect reliable, verifiable high-precision GNSS coordinates and associated metadata. Currently, most mobile GIS apps don’t differentiate between high-precision external GNSS receivers and the 3-meter GNSS receivers, or even the tablet or phone’s internal location service, but Collector will change that.
This update for Collector will have a couple things that put it above its competitors in relation to GNSS:
- Ability to connect directly to an external Bluetooth GNSS receiver to view the satellite metadata inside Collector without leaving the app.
- On-the-fly datum transformation for collected data points (i.e. NAD83 or variants of).
- Ability to shift Collector’s map imagery to correctly display user location when using an RTK network sending real-time corrections in NAD83.
- Ability to store satellite metadata like Estimated Accuracy, Correction Age, and Fix Type (GPS, DGPS, Float, Fix, etc.).
In detail, these new options will be plain awesome for field users.
Connect directly to GNSS receiver and view satellite metadata
This will allow users to avoid the constant toggling back and forth between the data collection app (Collector) and the GNSS receiver’s native interface app that shows the satellite metadata. This will be a real boon to users as it will allow them to stay inside Collector all day and avoid the annoyance of having to flip back and forth between Collector and the receiver’s native app to view the estimated horizontal accuracy.
You can see in the screenshot to the right that the estimated horizontal distance (2 cm) is always visible in the small box down in the lower left. If you tap that box, it pops up a larger box displaying additional metadata like vertical accuracy, fix type, correction age, number of satellites, and importantly the type of GPS Correction File currently enabled (NAD83 in this example).
You can also see that the feed from the external GNSS receiver is live. To show this, both Collector and Eos’ Tools Pro apps are open side-by-side. You can see the DGPS sub-meter (non-RTK) horizontal accuracy is 36 cm and vertical is 62cm in both apps.
On-the-fly datum transformation.
The inability to set the datum for the collected data, or viewing existing data, has long been a less than desirable aspect of mobile GIS apps. Mobile GIS apps have always recorded location data from external GNSS receivers in WGS84 (ITRF08) datum, which matches the aerial imagery utilized by the app (usually Apple Maps or Google Earth). This way the user’s location cursor in the app mostly matches the aerial imagery and their surroundings. Unfortunately, this has always meant that GIS staff has had to transform their existing project data from whatever datum (usually a variant of NAD83 or NAD27) to WGS84 before loading it into their mobile GIS app. Then after field data collection was completed, users had to transform the WGS84 data from the mobile GIS app back to the particular datum employed in their GIS. Annoying for GIS staff, and a cause of a lot of confusion for users that don’t have dedicated GIS staff.
This Collector update will take this annoying workflow out of the equation. Now the user can set the desired datum for the GNSS receiver right inside the Settings for Collector.
Shift Collector’s imagery to correctly display user location with RTK
As mentioned above, mobile GIS apps show the user’s location based on data from the GNSS receiver with the location cursor placed over map imagery in datum WGS84. This is problematic for RTK users as they are receiving real-time corrections for centimeter accuracy in NAD83. This has always caused a 4 or 5-foot shift in where the cursor shows the user on the aerial image and where the user is actually standing in the environment. For example, if an RTK user stands on a street corner and looks at their cursor location on the map, it looks like they are standing in the street.
Going back to our mailbox example, we establish a NAD83 datum conversion in the Settings for Collector. The RTK corrections for the Arrow 200 are in NAD83 datum and the Collector app is set to default (WGS84). You can see that even though the Arrow 200 is set up on the south side of the mailbox, it appears on the imagery to be shifted 3.7 feet to the southwest because the map “thinks” the data is in WGS84 when it is actually in NAD83.
Now, we go into Settings in Collector and switch from the Default GPS Profile to the NAD83 GPS Profile we created earlier (below left). Now everything will work with RTK corrections in NAD83 from the Arrow 200. Then we go back to our map and we can see that the imagery now shows us lined up on the south side of the mailbox as it should (below right).
Esri has been chasing the rest of the mobile GIS app world for a long time. But, with this summer’s update that adds on-the-fly datum transformations, Collector catapults itself above almost all competitors in the GNSS interface category. We are excited to spend more time in the Collector workflow with RTK equipment but hope that Collector continues to develop additional functionality to keep up with competitors that have more robust data forms.