Measurement Accuracy Between Points

I have an application that requires measurement of the distance between a manually plotted point and various points on a manually plotted range and bearing line using a second range/bearing between the two points as the measurement tool. Typical distances to be measured will be about 12 nautical miles. Questions:

  1. What would be the expected accuracy of the measured distance. (high accuracy is needed)

  2. Are there system factors that would effect that accuracy.

I would think it would be dictated by the accuracy of the chart and the ability to pick precise locations on a chart whose scale may be 1:125,000 or greater. It measures to the nearest 1/1000th, or about 6 feet but the scale of the chart probably won’t let you pick a spot to the nearest 6 feet. The only way to get 6 ft accuracy would be to take GPS coordinates at each location (assuming 6 foot accuracy on the GPS), set each as a waypoint and determine the distance between them…


Tom - Thanks for the response. For the sake of clarity I’ll simplify the scenario. I enter two “New Points” into CE based on coordinates from a third party source that are approximately 12 NM apart from each other. I then use a new “Range and Bearing” using the snap-to tool to place the end-points. What accuracy should I anticipate for the resulting CE displayed range and bearing. This is all independent of the background chart.

That sounds like a Brad question. It’s all in how they calculate the distance between two points. I suspect they assume both points are at the same elevation so the distance is always the horizontal distance and not the slant range. I’ve done this using Google Earth and setting two points. Precision and accuracy were not an issue the way I intended to use the result, I was more interested in determining where I had to be to have line of sight over a mountain at a given elevation angle. There are on line calculators that will give you an answer but I suspect the accuracy is determined by accuracy of how the coordinates of the points were determined in the first place. In the farming community where I live, some farmers are subscribing to paid services that provide GPS accuracy on the order of centimeters and let the autopilot drive the machine doing whatever needs to be done. To do that, they need to accurately (within centimeters) determine the corners of the field. Get that wrong and you might take plow up your driveway while watching TV in the cab.


For an exceptionally accurate distance measurement, compute the distance between two lat/long locations using mid latitude sailing with Clarke’s Spheroidal Correction applied. Tom Collins

While Coastal Explorer’s range/bearing lines are calculated with an extremely high degree of accuracy, the precision of their use as a measuring tool is limited to the values they display which are typically 1/1000 of a NM for the range and 1/10th of a degree for the bearing.

Since 1/1000 of a NM is about six feet, that’s about how much accuracy you can expect. For most uses of Coastal Explorer, that is more than adequate, and given that you’re talking about ranges of 12 NM, it’s probably accurate enough for your use but that’s up to you… (If it isn’t, you could switch to meters which will change the displayed precision to 1 M essentially doubling the accuracy of the display.)

One other thing to consider is that Coastal Explorer can calculate ranges and bearings using either rhumb line or great circle calculations (Settings > Measurements > Sailing), but I don’t think you’ll notice a differences around 12 NM.

Brad - I was pretty sure that was the case. My application is to create dead reckoning tracks for USN vessels based on the deck log’s three daily fixes and the in-between logged course and speed changes. The accuracies are vastly better than is required for that use.
Thank you for the clarification and to Tom for forwarding my initial post to you.