The companies offering these options have done nothing more then get the tax assessor maps off line or, bought the spec print books with map and landowner info available for purchase by anyone. Then took the time and effort to put it into a program for our use in the field. These aren't surveys copied. They are drawings a county tax assessor has drew setting in his office based on survey programs, deed calls, aerial photo's, etc.....
So, in areas that surveyors of old did a crappy job, surveyors weren't used to break up land when sold in the past, old non descriptive deeds, etc....., these mapping programs can be off substantially. In cases where credible work was done by a good surveyor, good deed calls are present, etc..... these programs can be extremely accurate. They aren't the gospel but, its as good as you can do without a known modern survey of all the land.
WV Mountaineer is on the right track but there's a little more to it.
He's correct that the lines and corners are based on tax maps but not all tax maps are drawn by the assessor and the accuracy of tax maps can vary widely across the country.
I'm a County Surveyor and my office was responsible for creating the first GIS parcel map (aka tax map) in our County and we continue to maintain it. Our tax map is based on the actual section corner monuments that are in the ground and those corners are the framework on which the map was created. Those corner monuments were located with extremely accurate surveys and are the basis for the lines and corners on our tax map. We also use current surveys to tweak and adjust our maps so they are as accurate as possible. As a result, our tax maps are typically within about 3 tenths of a foot (3 1/2 inches) or less from an accurate survey. Sometimes they're too accurate but that's a whole other topic...
The accuracy of assessors tax maps can very widely across the country and the key factor is whether or not a surveyor (typically a County Surveyor) was involved with the creation of the original base map. If a surveyor was involved with the original map, it will usually be based on actual surveyed section corners (or other monuments in the states that don't have section corners) and the map will be pretty accurate.
It's not so much that the old surveys/surveyors were bad but more about how the survey information was used to create the map.
In some counties and states the GIS folks and the assessor may have had a need to push their tax mapping program forward before they had the time to complete the massive and expensive survey. Or they may have believed that they did not need a prior survey. In those cases, the tax maps may not be very accurate. The assessors don't really care about accuracy; they just need an approximate map that they refer to for tax collection and it was never intended to be used as a survey or for hunters to know where property lines are.
Some tax maps are based on the location of section corners that were digitized (electronically scaled) off of old USGS Quad topo maps. Those maps can be very inaccurate.
Later when the tax map becomes part of a County-wide or state-wide GIS and the need for greater accuracy is realized, the work required to perform the necessary survey and redraw the entire map can be so large and expensive that it is never done and they just live with the inaccurate maps.
So when companies such a ONX acquire the often free parcel information for the entire country they are stitching together a mish-mash of data with widely varying degrees of accuracy.
You could be hunting in one county where the tax map was based on an actual survey and the property lines could be right on the nuts, but then you could cross over the county line to a county with less accurate parcel information and you could be way off.
Some states like Wisconsin had a state-wide initiative to have accurate GIS/parcel maps and the mapping app info is probably pretty accurate there. I work in Minnesota and many of the counties there have accurate GIS/parcel maps but some of them not so much.
Some of the really rural areas with not a lot of money to spend on surveying and mapping might have some really crude and inaccurate data. Many counties across the nation don't even have County Surveyors. In those counties the assessor and/or GIS folks often prepare maps without input from a surveyor or without even understanding the benefits of having a surveyor involved. Then there are the GIS folks who believe that surveys are totally unnecessary.
Sorry for the long explanation. I'm not saying that there's anything wrong with these mapping apps, it just might help to understand the basis and original intent of the data.
So from a "common Joe hunters" perspective, where can these "free" maps be located?
Example: go to Colorado and type in GIS mapping, would that get us pretty close to the points that we could then enter into our GPS's - do you think that would that work?
What are some other mapping sources?
Inshart, they're not really free "maps" but rather it's digital data that is usually free because it was typically developed at taxpayer's expense and has already been paid for by the public. Although some data is deemed to have commercial value and you may have to pay for it.
The same goes for aerial photography. Most aerial photography that has been acquired by federal, state, county and city agencies has already been paid for by taxpayers and is often available in digital form at little or no cost.
Also the same with all of the USGS topography data for the entire United States, it's all available for free and has been downloaded, repackaged and sold by the companies that offer custom topo maps or have topo layers on their websites or apps.
The data itself is typically free or minimal charge and in many areas there are laws that require it to be free. However if you require the data to be printed out in map form you can be charged for the map to recover the cost of equipment, supplies, etc. Or if you're one of the companies that repackages the free data into your own unique custom product, you can charge whatever the market will bear.
You typically need a GIS mapping software package such as ESRI ArcGIS or ArcInfo, etc. and the technical knowledge necessary to turn the free/cheap digital data into maps or aerial photos.
Private mapping app companies and even Google Maps, etc. know how to contact the government agencies and acquire the data for little or no charge. Then their skilled technicians use the appropriate GIS mapping software to stitch together the data into seamless maps and create all of the various layers such as recent wildfire burns, land ownership, etc.
These map app companies are not out there surveying or tracing the boundaries of wildfire burns or any other layer they have available on their app, they're simply acquiring the data from the appropriate government source and adding it to their app.
I'm a Land Surveyor and Division Manager, I'm not really a GIS mapping expert but I have mapping experts working for me and I know enough about it to understand the basics of it.
I'm afraid that in order to create your own maps from the available data you'd need some expensive software with crazy annual license fees and considerable GIS mapping experience.
That being said, there is a surprising amount of map data available out there for free. For instance, all of the USGS topo quads are available for free at many different scales and you don't need any special software or skills to download it, all you need is access to a map plotter in order to plot the maps. That's why I never use the companies that create and sell custom topo maps, I just go to the USGS website and download the USGS topo quad maps for free.
Back in the house I went to google earth to add it to my list of stock tanks, guzzlers. When I inputted it into google earth I found that it was a stock tank I had found by looking for water sources. My waypoint showed up as exactly where I was standing when I created it.
Keep in mind that in many cases a winding creek serves as the property line and the plat only shows the sideline trees. The sideline trees are only references and are not the actual property line. High dry ground is a lot easier to survey.
Regarding GIS software, I hate ESRI and their expensive licenses. Currently I use QGIS and you can pretty much do anything with that as you can with ESRI, and it is open source and free. Highly recommended if you want to do GIS work.