Why GPS is off, and by how much
The blue dot lands on the wrong side of the street. The position drifts while you stand still. A GPS position always carries error, but it comes from a short list of places, and some of them you can fix yourself. Here is that list, sorted into what is in your hands and what is not.
Position is decided by measuring distances
Each satellite broadcasts the time and where it is. The receiver works out how long the signal took, and turns that into a distance. Four satellites are enough to fix a position: four rather than three because the receiver's own clock error has to be solved at the same time.
So anything that distorts the distance becomes error. Everything below distorts a distance.
Five sources of error
The ionosphere. A charged layer between roughly 100 and 1,000 km up delays the signal. Uncorrected that is tens of metres; with the simple broadcast model applied, several metres remain. This is the largest source today.
The atmosphere. Water vapour near the ground delays the signal too. Humid days are worse. A few metres.
Reflections. Signals bounced off walls, glass, the ground or a vehicle arrive late and read long. In a city this is almost always why the dot sits on the next street. Open sky is not enough if there are reflecting surfaces around you.
Satellite geometry. Satellites spread across the sky give a sharp fix; satellites bunched together give a vague one. The measure of this is DOP, and smaller is better.
The satellites and the receiver. Satellite clocks and orbits carry a metre or two of error. Receiver noise and antenna quality matter as well.
How far off, in practice
A few metres under open sky; five to ten in a suburb; tens of metres between tall buildings. Indoors you get no fix, or a badly wrong one.
Vertical error runs 1.5 to 2 times the horizontal. The reason is simple: the satellites are all above you, so there is no geometry from below to pin the height down.
What you can fix yourself
In order of effect. Get to open sky. Stepping out from under a building, a tree or an overpass changes more than anything else on this list.
Change how you hold it. Inside a bag, deep in a pocket or behind metal, reception falls away. Do not cover the face that points at the sky.
Wait a moment after switching on. It takes tens of seconds for the position to settle.
Use more than one constellation. GPS plus QZSS, GLONASS, Galileo and BeiDou more than doubles the satellites in view and keeps the geometry from going lopsided.
Choose a dual-frequency receiver. With two frequencies the ionospheric delay can be computed away, and reflections hurt less. On your next purchase this is the difference that matters most.
The wall a single receiver cannot pass
Do all of that and a standalone position still sits at the level of metres, because the cause is not in your receiver but in the sky the signal came through.
There is one way past it: compare against a receiver at a known position. A nearby receiver sees the signals through the same sky, so it carries the same error. Subtract that and the common part disappears. This is RTK, and it brings the error down to centimetres.
You need a receiver that supports it and a source of corrections: a public base station, a paid service, or in Japan a correction broadcast from the QZSS satellites.
Start by looking at what is around you
Before buying anything, find out whether there is a usable base station near where you work. Accuracy falls with baseline length, so the question is whether one sits within about 10 km.
KUON GEO polls the world's public base stations every ten minutes and lists them with their uptime. On the map, each station carries 10 km and 20 km circles, so you can see at a glance which one your site falls inside.