What one needs to understand about topos is that all rural and unpopulated areas are generally derived from aerial photos, with only those areas that are actually surveyed being applied to the topo map using overlay technology, while the rest is either digitized, or transcribed manually. Accuracy depends on two factors....The accuracy of the aerial photography altimeter readings and readout, or human error. Aircraft are subject to minor fluctuations in altitude due to air currents, resulting in height differences that are generally in the 12 to 30 foot differences in altitude, but in severe cases can be much more than that. Satellite imagery is usually more accurate, but that too is subject to minor differences. Even large areas manually surveyed are not always accurate, as measurements in open undeveloped areas are taken in 100 foot sequences as a grid, with the surveyor estimating the curvatures and elevations between readings.
That's why, whenever a new structure is to be built, a survey is required using a 25 foot grid, unless the subject property is basically flatland, in which case the surveyor will usually use either a 50 or 100 foot grid. In the case of highway construction, the right-of-way is more rigidly surveyed and staked with two grids, one in the direction of travel, the other diagonally. In the process, the Highway Engineer takes sub base core samples every 50 to 100 feet, based on sample findings, which is extrapolated to the survey indicating appropriate depths and make-up of sub strata.
But there I go again, getting far too technical. We're not building freeways here, but now you will have a better understanding of what's being done with the lands around you when you see those survey crews out along the roads.
This is cool!! Thanks.
JMJ...T Y !
Any help is welcome to help us AMERICANS when the need arises. I am 74 and scared to death of this man in Washington. God save America
Thanks Twana! I think I can use these for my Logistics Project.
With a bit of research you can find quad sheets and photo maps for many states on line and you can download them for free. Problem is, that in order to print them you need a large format printer such as is used by drafting, graphic arts people, etc. I know this because we used these all the time doing property surveys. Here's the address for Arkansas. http://www.geology.ar.gov/geologic_maps/quad24k_map.htm
At one time I had all of the 7.5 degree quad sheets for the state.