How long should I average a point or vertex?
Great question! I get this question all the time. My canned answer is 20-30 seconds. But being more of a solution provider than just a reseller, I like to probe a little further. How long will it take the person collecting the data to enter the attributes? 60 seconds? You might as well be averaging for 60 seconds then. If there are no attributes then best practices dictate 20-30 seconds.
Why do we average?
Trimble mapping hardware converges to a solution/position every second. By averaging a group of these positions the software can do a best fit and you will see great gains in your post-processed and real time accuracy. Think of it this way, the point of the rod, or the plumb line from the receiver is the bullseye of a dart board. Every second the receiver converges to a position, it “throws another dart” near the bullseye. It is best for the software to take an averaged position of 20 or 30 darts. Please demonstrate this to yourself. Zoom way in; as close as you can in the map, view and watch your position move around while the receiver is standing still. This is why we average!
Recently, I delivered a 50cm Geo7X to a new client. We met at a job site in La Jolla, California. They believe they are losing market share to larger consulting companies that, in addition to marking out with paint, are providing GIS, CAD, and spatial data on the existing underground infrastructure as value added to their clients. We proceeded to collect some linear features. “How long should I average these vertices?” 20-30 seconds. I immediately saw the company’s owner calculating how long it would take to shoot 400 vertices. Let’s see, 400 vertices/points, 30 seconds each, 6000 seconds, divide by 60 for minutes, again by 60 for hours…..3.5 HOURS! WHAT?! He had two more jobs that day.
How long you average is always a cost benefit analysis. If you find yourself shooting (or needing the efficiency to acquire) more than 100 vertices/points a day, you may want to consider a VRS (Virtual Reference Station) real-time service.
VRS real time corrections enable your mapping receiver to hit spec in real time, in the palm of your hand. So EVERY time the receiver converges to a solution (once per second) you will be hitting the accuracy specifications of the receiver. No more averaging. You only need one second and you are ready to move. Additionally, it eliminates post processing and waiting for base stations to post data.
What is a VRS Real-Time Network and how do I sign up?
VRS is an array of reference stations that calculate the corrections for your position and send them over a network to the receiver.This is very much like an RTK survey, only the corrections are coming over a network rather than a radio wave. In fact, many survey grade workflows utilize the same VRS networks. The Geo7X’s have a dedicated SIM card slot for internet connectivity, but many of our clients just hub this off their phones or other WiFi hotspot devices. Enter the IP address, user name, password and you are ready to collect without averaging.
Is a VRS Real-Time Network subscription right for you?
The answer to that lies within the yearly cost and the required efficiency of the person collecting data in the field. The more positions you need to acquire a day, the more it makes sense, and actually SAVES you money! The City of San Diego subcontracted with a company to monitor and collect storm drains. They were doing nearly 100 per day without VRS corrections. When they started utilizing VRS corrections they collected 500+ per day. If you feel that averaging positions is costing you money in your staff’s time (in the field), or you would like to increase efficiency of data collection, talk to a CSDS Mapping representative or our System Administrator about the benefits of VRS real time corrections. They can answer questions regarding how much a CSDS Real-Time Network subscription costs, what network you fall in (northern or southern California), how much data is transferred and if you can set up your own real time network.
Jon Gipson, PG, GISP is a Solutions Specialist with our Mapping & GIS Division. He supports CSDS customers throughout San Diego and southern California.