Q:

Why can't lines plotted on google maps suffice for legal determination of property boundaries?

If there is a way to calculate a maximum error in a line plotted on google maps (road map, or satellite view) , then that should be admissible in a court of law, shouldn't it ?
For example, my deed describes my property boundaries starting from a "point of commencement" and then gives distances and angular measures to eventually arrive at the POB and similarly then continues to outline the actual boundaries.
If I locate the "point of commencement" I could, with a bit of coding, trace exactly those lines on google maps.
Is there some way to estimate the maximum error in real world distances of such lines as shown on the map from their actual location?

A:

I'm no lawyer, but i am 99% sure the very short answer to your question is "no".
The legally admissible boundaries are recorded in the cadastral record which you can obtained from city hall. That record is generated by surveyors who do the similar process to what you describe (start at an origin, draw lines at certain angles etc) but with real life measurements. 
Any software generated boundaries will be prone to different types of errors which will make them inadmissible. Courts like dealing in absolutes. There exists a reliable method of doing this, and its how they generate the  cadastral record.
Even with highly accurate lidar or remote sensing images, when it comes time to plot out the property in real life, the process requires professional surveyors who will then sign off on the results.  

Is there some way to estimate the maximum error in real world distances of such lines as shown on the map from their actual location?

You would have to draw the polygon in your software, get some coordinates from the software (the corners of the lot for example), and then go to the property and measure the real boundaries with a high accuracy GPS device ($$$) and then compare the two sets of readings. Doing this for one property would give you an idea of the margin of error of the process, doing it for multiple properties will help you dial in the margin of error. That would be the process you are asking about in this above quote... but it still wont be admissible.
You could compare those results to the cadastral files to guage the margin of error, but once you have the cadastral file i'm not sure i see the point of making another record/file. 
the context might greatly change what is admissible. Trying to move a boundary between two houses is very different than say, trying to estimate the surface area of a very large property. 

A:

Anything related to land or property is governed by a Professional Licensed Surveyor (PLS).  Whether it’s having the dimensions checked or verified, disputing a property line with a neighbor, proposing a new development or buying land, etc.  
A document known as an ALTA Survey is required at the expense of the landowner/buyer/seller before any legal proceedings occur.  This is a detailed “map” of the property, complete with very specific language describing the limits of the plat. 
You can find examples of these documents through many county GIS websites, often under the tax information link.  The lines you see on Google Earth or GIS software mean nothing legally but are for information only.  
In addition, ONLY a PLS and nobody else is qualified to physically place the property pins at the lot corners. 

[this pertains to the U.S.]

A:

It's actually a good deal more complex than that. A surveyor does not take the geometry in your deed at face value. He or she will compare it with the geometry of the adjoining tracts, and if inconsistencies are found, will trace back the title to see which chain of title is "right." And then he'll start the field survey. As it turns out, the location of monuments (physical objects) called for in the deed trump those courses and distances if they disagree. And senior rights (older deeds) trump junior rights (younger deeds). So the jigsaw puzzle of parcels has not 3 dimensions, but 4, with time being the fourth. Earlier grants control younger grants in the case of inconsistency.
An ALTA survey is usually used for a commercial land transaction. It is not necessary for court action. In short, the surveyor is the one who makes that call not only because he's pretty good at measurement, but because he understands boundary law. That has as much (or more) these days to do with the boundary location than the measurements. I'm a licensed surveyor; take it from me.

