the Degree Confluence Project

United States : Colorado

4.4 miles (7.0 km) SE of Erie (Weld), Broomfield, CO, USA
Approx. altitude: 1589 m (5213 ft)
([?] maps: Google MapQuest OpenStreeMap topo aerial ConfluenceNavigator)
Antipode: 40°S 75°E

Quality: good

Click on any of the images for the full-sized picture.

#2: North #3: East #4: South #5: West #6: GPS/Data Collector Shot

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  40°N 105°W (visit #35)  

#1: Area

(visited by Sam Gallucci)

27-Apr-2017 -- I had always wanted to visit a degree confluence using survey-grade Real-time Kinematic GPS. It so happened that last week I began the task of creating an ALTA map/boundary survey for some large parcels of land in the same subdivision that 40N 105W is in. While not directly related to my project, the area of the confluence might end up being a grassy lawn for the future University of Colorado Health medical campus I've seen some plans not sure how old.

This confluence is situated in Section 4, Township 1 North, Range 68 West of the 6th Principal Meridian if you consider the U.S. Public Land Survey System (PLSS). It is governed by the City and County of Broomfield, a consolidation of a city and county.

I arrived in the morning and set up my base GPS receiver and external radio equipment over Broomfield GPS Control Point "LUCY". "LUCY" is the closest control point to the degree confluence. Using my data collector, I already had the job set up for the base to broadcast relative positions using the published latitude and longitude of "LUCY". I then used my "rover" GPS receiver to create a Differential GPS method in real time. RTK. Having a known point nearby allows the system to cancel out any atmospheric issues.

After checking into another Broomfield control point I continued my work collecting positions of land corners. That afternoon I decided to take a quick break so I drove to just north of 40N 105W and parked at the side of State Highway 7. I simply created a point in my data collector with latitude and longitude for the confluence and then told it to stake out the point. It took me south of a barbed wire fence and into typical Colorado barren field with native grasses and weeds. There were some underground sewer line markers nearby. Further to the east was a sign for the future medical campus. I was hovering around 0.005' north and 0.010' east. Nudged it a bit and began my observation using a 3 minute (180 epochs) setting. The screen shot I took is of the observed point after resolution of my 3 minute observation. What the collector does is averages the 180 observations.

I took my required photos and began to walk back to the truck. Then I realized that I could put some pink flagging at the top of two fence posts and observe them and get distances so that future confluence hunters could retrace the position and see how far off their GPS is. There is a flagged fence post to the west that is 37.86 ft to the northwest of the confluence while the other flagged fence post is 39.90 ft to the northeast of the confluence.

When I got back to the office I ran a points report which I include herein.


  • Point as-staked:
  • N 39°59'59.99999"
  • W 105°00'00.00000"
  • Distance to confluence 0.001 ft with 95% probability.

Published base LUCY:

  • N40°00'00.35831"
  • W105°00'41.29278"


  • Trimble R8 model 4 GPS receiver (base)
  • With external radio
  • Trimble R6 model 4 GPS receiver (rover)
  • Trimble TSC3 data collector

 All pictures
#1: Area
#2: North
#3: East
#4: South
#5: West
#6: GPS/Data Collector Shot
ALL: All pictures on one page