You may not realize it, but GPS accuracy varies from day to day. Satellites occasionally go on the fritz, but the biggest issue is the arrangement of the constellation relative to your position.
You see, GPS satellites aren’t in geosynchronous or geostationary orbits, but are instead in medium earth orbit (MEO), giving them an orbital period of 12 hours. GLONASS and Galileo use similar MEOs. You can see these in the chart below:
Good days and bad days
What this means is that the constellation is always changing relative to your position on the face of the earth, as can be seen in the graphic below and to the right. Note the changing number of visible satellites.
This impacts not only surveyors and high-precision users, but others who want the best accuracy, be it for mapping trails, marking boundary lines, hiding geocaches etc.
Consumer-grade GPS won’t give you the accuracy of post-processing equipment, but you should be able to get accuracy of 10-30 feet, IF you choose a good satellite day (and time of day, which is important too). Which brings us to…
Satellite prediction tools
There are a number of tools available online to predict good satellite days and times of day. This CalSky satellite prediction tool is one that worked well for me in researching this post.
It allows you to choose whether to show GPS and/or GLONASS coverage and specify the minimum elevation of satellites in degrees above the horizon. In the chart below, the bars (|) and X’s indicate visible satellites. The PDOP is also given, with lower numbers being better.