Last week was spring break (for high school and college) for my kids. We decided to drive to San Francisco and the surrounding area to do a little sightseeing. It’s a beautiful place. This is a view from our 3rd floor room in the hotel, looking over the bay.
Here’s a view from the Marin Headlands overlooking San Francisco Bay and the Golden Gate bridge.
Of course, while traveling, I usually manage to work in some GNSS activities.
The first stop was Autodesk, the makers of AutoCAD and other engineering, design and visualization software in downtown San Francisco. AutoCAD occupies 100,000+ square feet at One Market St. in downtown San Francisco and another 20,000+ square feet at Pier 9 right on the Bay. How anyone gets work done with an office on a San Francisco Pier is beyond me. It’s buzzing with people and activity, including a shuttle to the famous Alcatraz Prison, which we enjoyed.
The Autodesk meeting is deserving of an article in itself, but I’ll keep it short with bullet points for the purposes of this article:
- AutoCAD 2014 includes a datum/coordinate system library for mapping/surveying users. This is new in AutoCAD.
- Infraworks (introduced last year) was built from the ground up with a new workflow for engineers and planners (and surveyors). Most people have never heard of it. It can do things that AutoCAD can’t, such as managing surveying data for large-scale projects. Think BIM (Building Information Modeling).
- Model Builder (just introduced), is a tool to build quick and dirty 3D visualizations using data from Autodesk’s cloud service.
- Autodesk 123. This is a really cool free app you can use to create 3D models using your own images. The images can come from smartphone pictures or images you already have. It’s a really cool app.
- AutoCAD 360 (formerly AutoCAD WS). First of all, any Autodesk product with 360 in the name is a cloud app, whether it’s mobile or desktop. I’ll focus on the mobile apps. There are two AutoCAD 360 mobile apps: one for Android and one for iOS. The mobile apps are free tools that allow you to take AutoCAD drawings in the field. There are also Pro versions available on a subscription basis.
Last week, I had a chance to use AutoCAD 360 in the field with RTK. It was a last-minute exercise that I hadn’t planned on, so my expectations were set so that even if I couldn’t get it to work, at least it would be a solid learning experience.
The goal was to receive 1-2 cm RTK GNSS positions on an Android smartphone running AutoCAD 360 using a public (free) RTK base station. I knew I could access the free RTK base via PBO real-time streaming because I’ve done that before. However, I didn’t know, or have experience in two areas:
- Accessing RTK base data via NTRIP on an Android device.
- The ability of AutoCAD 360 mobile app to consume GPS data.
For the Android device, we used a Samsung Galaxy Note. It’s a smartphone, but also a tablet with a 5.7-inch color touchscreen.
The first challenge was the Android utility software needed to access the RTK base. NTRIP (Networked Transport of RTCM via Internet Protocol). As I’ve written in previous articles, there are lots of free RTK base stations (330+) in California. To access them, all you need is internet connectivity and an NTRIP program to manage the connection to the RTK base. For Windows and Windows Mobile, there are several free NTRIP software programs. For Android, it’s limited (but growing). I found a free Android NTRIP utility on the Google Play store. It’s very easy to install and set up. If you have your RTK base credentials (IP address, port#, login, password), if you have a Bluetooth RTK receiver, you can install the program and be running RTK within a few minutes.
Once I entered the RTK login credentials, I was presented with a list of RTK bases. The list of PBO RTK bases are all single-baseline RTK bases (not networked) so I needed to select the closest one to the project site. In this case, it was P178 (see the screen shot above). It was about five miles from the project site. At this point, I can see the RTK base data streaming on the Samsung Note tablet. I didn’t mention before, but I had already Bluetoothed the Samsung to a small RTK GNSS receiver. Once the RTK base data starts streaming, the RTK GNSS receiver goes into FLOAT mode and heading for FIX (1-2cm precision).
At that point, we (I wasn’t operating AutoCAD 360 on the Samsung) started AutoCAD 360 on the Samsung Note tablet and loaded a drawing that we’d planned to use. Following are a couple of screen shots from our exercise.
It took a minute to figure out how to”turn on” GPS in AutoCAD 360 (we were all newbies), but once we did, our position showed up on the drawing where we expected it. By this time, we were getting an RTK FIX position from the RTK GNSS receiver. We were getting 1-2 cm precision in a native AutoCAD drawing, in real-time, in the field, on an Android smartphone. I was impressed.
We were ready to start our accuracy testing. Our accuracy testing consisted of two parts:
1. To test precision, take RTK shots on two points and measure the distance between the two with a tape measure. We did this several times.
The results were as follows:
- P1 – P2. Measured distance: 20′ 9.75″. RTK distance: 20′ 9.0″.
- P2 – P3. Measured distance: 21′ 11.5″. RTK distance: 21′ 11.75″
- P1, P2, P3 were about 12 feet east of a 18-20 foot high concrete wall.
- B1-1 – B1-2. Measured distance: 6′ 3.0″. RTK distance: 6′ 2.25″.
- B1-1 and B1-2 were 15-18 feet from the 18-20 foot high concrete wall.
- Lt-1 – Lt-2. Measured distance: 12′ 2.0″. RTK distance: 12′ 3.0″
- Lt-1 and Lt-2 were on top of a platform with no substantial obstructions.
Lastly, we took a shot underneath a platform with greater than 50% of the skyview obstructed. It didn’t hold RTK in that environment and I didn’t expect it to. The precision was 5 feet (DGPS).
2. The second test was to test accuracy by taking an RTK shot on a survey marker that had published State Plane Coordinates in NAD83/2007 epoch 2007.0
After recording an RTK FIX shot on the marker (albeit I was holding the antenna so I expected a little slop), we compared our result to the survey marker coordinates. Not good…3.0 feet difference.
My first suspicion was that the RTK base was referenced to ITRF, so there would be significant difference between the two coordinate values. No dice. I adjusted the RTK GNSS coordinate to NAD83/2007 (2007.0) assuming it was referenced to ITRF08. The adjusted coordinate was further than the original (6.95 feet). That wasn’t the problem.
My second thought was to double-check what the PBO folks used for a reference position for there RTK bases. They confirmed ITRF08 current epoch. However, after talking to a few people familiar with PBO sites (RTK Network operator and Mark Silver), they suggested to run an OPUS solution on the PBO RTK base and compare it to the reference coordinate being used by the PBO RTK base. Sure enough, there’s a 6.40 feet difference between the 24 hour OPUS ITRF08 coordinate and the ITRF08 reference coordinate being used by the P178 RTK base.
It still doesn’t reconcile the difference we saw between the RTK GNSS coordinate and the survey mark, but I’m still trying to confirm which epoch date the PBO RTK base is using. In California, tectonic plate movement is significant. In that area, the ground is moving 1.7 cm north and 3.4 cm west each year, so the epoch date of the coordinate is significant, especially if the epoch date is 1997.0 or 2002.0. However, that doesn’t prevent you from using RTK Bases like P178 and “localizing” to NAD83/2007 or whichever datum your data is referenced to.
Thanks, and see you next time.
Follow me on Twitter at https://twitter.com/GPSGIS_Eric