High-accuracy mobile data collection in NAD83 CSRS

WGS 1984 Web Mercator is the default mapping projection used in virtually all major online map providers. However, when collecting data with Esri’s mobile apps, you may be required to generate high-accuracy centimeter-level precision. In Canada, collecting data using the NAD 1983 CSRS datum is more precise because it is specifically tailored to the North American region, incorporating more accurate local measurements and adjustments. When horizontal centimeter level precision is required for your projects, Esri Canada has a variety of partners who provide GNSS receivers capable of collecting real-time kinematic (RTK) positioning. In this blog article, we will demonstrate how to collect and validate horizontal centimeter-level data collection using Eos Positioning System’s Arrow Gold RTK GNSS receivers in Sherbrooke, Québec.

Before diving into the details, if terms like "datum," "geographic coordinate system," and "projected coordinate system" leave you perplexed, I highly recommend perusing my colleague Sarah Sibbett’s blog series Your Guide to Canadian Datum Transformations and ArcGIS Online (see part 1, part 2 and part 3). That blog series provides a good base level of knowledge that this blog post builds upon. The primary focus of this blog is to highlight methods for gathering data within geographic coordinate systems beyond WGS 1984 Web Mercator, when utilizing an external GNSS receiver. In Québec, the provincial government’s horizontal control survey marker network is available as a Web Mapping Service (in French), which can then be added to ArcGIS Pro when planning your data collection (Image below).

As you can see from the figure above, we know that the provincial surveying network is based on the NAD83 (CSRS) (v2) datum. To demonstrate and validate the accuracy of our data collection while in the field, we will be using two horizontal markers as ground control points (see image below) while connected to the Québec government’s Ministry of Natural Resources and Forestry permanent GNSS reference station located in Sherbrooke, Québec. Establishing a connection to the permanent GNSS reference station enables the precise collection of centimeter-level data with real-time kinematic (RTK) correction. Note – not all countries, provinces and isolated regions have a free permanent GNSS reference station. You may need to purchase this service from third-party service providers (see here for more information).

In this blog article, we will showcase two separate workflows to collect high-accuracy data in Field Maps by showcasing how to prepare the maps in ArcGIS Pro.

1. First, we will create a standard Web Map in the WGS 84 Web Mercator (EPSG: 3857). We will then create a location profile in ArcGIS Field Maps to complete a datum transformation from WGS 84 Web Mercator (EPSG: 3857) to NAD 83 CSRS (EPSG: 4617), the corresponding geographic coordinate system being collected from the GNSS receiver.
2. Second, we will create a custom vector-tile basemap and map in NAD 83 CSRS MTM 7 (EPSG: 2949) directly in ArcGIS Pro and publish it to ArcGIS Online. By creating a map in the same geographic coordinate system as the incoming GNSS base station, we avoid the need for on-the-fly transformations in Field Maps. When high levels of accuracy are critical, it is better to project the data to a common coordinate system before editing due to scale distortions of Mercator projections near the poles. Transformations can also slow down the application and it is therefore recommended to avoid them if possible. We will also showcase how you can use your organization's drone images to generate project-specific imagery basemaps in a geographic coordinate system other than WGS 84 Web Mercator.

Preparing the WGS 84 Web Mercator map for ArcGIS Field Maps

To prepare the WGS 84 Web Mercator map for ArcGIS Field Maps, we maintained the basemap in WGS 84 Web Mercator (EPSG:3857). We then used the Add GPS metadata fields geoprocessing tool to add metadata on the hosted feature layer that will be used for data collection. This hosted feature layer was published in the correct projected coordinate system for the project (NAD 83 CSRS MTM 7). Before publication, we needed to ensure that the map possesses the correct datum transformation. This same transformation will be necessary for setting up the location profile in Field Maps.

Note – when setting up the GNSS Coordinate system, Field Maps only accepts geographic coordinate systems and not projected coordinate systems. The correct datum transformation between NAD 1983 CSRS and WGS 1984 Web Mercator Auxiliary Sphere in Canada is more than likely NAD_1983_CSRS_To_WGS_1984_2, as explained previously by my colleague Sarah Sibbett.

Preparing the NAD 1983 CSRS MTM 7 map for ArcGIS Field Maps

The default basemaps in ArcGIS Pro and ArcGIS Online are all in WGS 84 Web Mercator; therefore, if you want to publish a Web Map in NAD 1983 CSRS, you cannot use these basemaps and will need to generate your own in the datum and geographic coordinate system you want to use. Custom basemaps are best created as a vector tile package , so our first step was to generate one to be used as the basemap. This can be accomplished using the create vector tile package geoprocessing tool. The vector tile package was then uploaded to ArcGIS Online. For more information on how to create a web map in a coordinate system different from WGS 84 Web Mercator, you can follow the Esri tutorial Make a web map without Web Mercator. The vector tile package used in this project is located here.

Once the vector tile package is published to ArcGIS Online, you can use it as a basemap in ArcGIS Pro. We then created and published the layers used for data collection in NAD 1983 CSRS MTM 7 (EPSG: 2949) and added the GPS metadata fields. Once the web map was configured in the Field Maps designer, we were ready for field data collection. The map generated for this blog is located here. Since the incoming GNSS data and the web map are using identical datum’s, no transformations are required when setting up the location profile in Field Maps. In summary:

1. We generated a vector tile package (.vptk) in ArcGIS Pro in the correct datum and geographic coordinate system (EPSG: 2949 in this example).
2. We uploaded and published the vector tile package to ArcGIS Online/ArcGIS Enterprise portal.
3. We created a custom basemap in ArcGIS Pro from the published vector tile package.  
4. We created a map in the correct datum and projected coordinate system in ArcGIS Pro (EPSG: 2949 in this example).
5. We created feature classes in the correct datum and projected coordinate system for data collection in the field (EPSG: 2949 in this example).
6. We ran the add GPS metadata fields to generate GPS metadata fields for each feature class to be used for data collection.
7.  We published the Web Map with editing capabilities enabled for the feature layers to be used for data collection and configure the smart forms in ArcGIS Field Maps as necessary.

Note – If you have drone imagery, you can use your own orthomosaic to generate a custom imagery basemap in the coordinate system of your choice by generating a tile layer package. My colleague Céline Doré has captured a video on the process. An example used for this project can be found here, using a DJI Mavic Pro 2 for the imagery and SiteScan for ArcGIS for the flight planning and imagery processing.    

Validation process

When validating the precision of the incoming data, you must first ensure that your GNSS receiver is properly connected and receiving data in the third-party mobile application. In our example, we used the Eos Tools Pro (image below). In Field Maps, ensure that you have switched your location provider in your profile from the integrated provider (i.e., your mobile device’s GPS) to your third-party provider. In our example, we were connected to an Eos Positioning Systems GNSS receiver. Now that you are certain that Field Maps is connected to the external receiver, you can start collecting points to ensure accuracy. Having added the GPS metadata field enabled us to see in real-time the accuracy of the data being collected. In the image below, we see that we have the receiver’s name, the horizontal accuracy, as well as the validation that we are RTK-fixed.

Note – The connection process listed above is for iOS devices. Mock location will likely need to be activated for Android devices to override the mobile device’s internal GPS. See following instructions for necessary steps.  

Since the geodesic markers are often found below ground, we used a combination of the high accuracy of the Eos Positioning Systems GNSS receiver and a metal detector to locate the markers. A shovel was also necessary to dig out the marker as seen in the images below. We would like to thank our partner Groupe Trifide for helping us with the intricacies of locating the survey markers in Québec.

Comparing horizontal accuracy

The table below shows the comparison of the various methods used to test the data collection process. The official geodesic marker position rows represent the government’s official location for the markers. As you can see from the table below both location profiles in Field Maps were capable of collecting data at centimeter-level precision with some variation from 0.2 cm to a maximum of 20 cm. This can be explained by multiple factors.

1. First, we did not have an actual range pole during the data collection process and improvised with a non-leveled hockey stick.
2. Second, multipath interference such as trees, buildings, and temporary cloud cover can impact the precision of the RTK signal and therefore impact the precision between two points being collected at the same location.
3. Finally, distance from the GNSS base station itself can also lead to a few centimeters difference.  
   

Closing remarks

The purpose of this blog is to give you the tools to prepare you for high-accuracy data collection with Esri’s mobile applications. In this example, we collaborated with Eos to demonstrate the workflows but Esri is compatible with a variety of third-party providers.

Before going out and collecting crucial data for your organization, it is a good idea to do this type of exercise to ensure you are using the proper workflow for your business case. Most provinces have their own geodetic control networks. Then, ensure that your receiver is properly connected to the RTK base station you will be using. A list of public and private networks can be found here. Once you’ve located the proper URL and port to access the RTK network, ensure that your third-party application is properly fixed to the network and demonstrating high-accuracy data. If not, contact your third-party vendor. Next, in Field Maps, change the default location provider to the third-party receiver you have previously connected with.

Next, set up your location profile (we have found that this is where most of our clients appear to struggle). The GNSS Coordinate system is specifically the geographical coordinate system of the GNSS station. Our recommendation is not to over complicate things by using the various datum versions (i.e., NAD 1983 CSRS v4 or NAD 1983 CSRS v7). If you choose to work with a specific datum version, you may find it helpful to work with an organization who is knowledgeable about geodesy in Canada. In our case, we worked with our business partner S.E.A. Graphics Inc. In this case study, the Québec Government’s geodetic control network is primarily in NAD 83 CSRS (v2) (EPSG: 8235), however, we used NAD 83 CSRS (v1) (EPSG: 4617) for the sake of simplicity and because our experienced Land Information Solutions team knows it to be accurate. This simplified workflow may lead to a very slight offset (varies by region over a few centimeters); however, depending on your needs, this may be more than acceptable. Not being overly precise with datum versions also simplifies the use of transformations in Field Maps when using WGS 84 Web Mercator basemaps, as it allows us to avoid having to generate custom datum transformations and configure them in Field Maps.  

Finally, start collecting points in Field Maps and ensure that the GPS metadata fields are representative of the accuracy you are expecting. Back from the field, compare your results with that of the known location from the geodetic control networks. Once you are confident with your workflows, you are ready to hit the field for your data collection!

This post was translated to French and can be viewed here.