When surveys are done, is it standard or even required to take elevation into account? For example, if you take two points on a slope, the distance between the two points on the surface would not be the same if measured on an ortho photo, correct? So do modern surveys account for elevation so that when laid out over a flat ortho photo, it will appear more accurate? If so, has this always been standard surveying practice, or was there a time when elevation wasnÛªt accounted for and only the distance between the two surface points were measured. When did this become standard or even required practice for licensed surveyors?
The reason I ask is I recently started a job working on tax maps. This is my first job doing anything with mapping, and I come across many older surveys from decades ago that donÛªt fit very well over the photos and I have to make a lot of adjustments to fit within the roads, fence lines, rivers etc, that they should be lining up with. While IÛªm sure there are many factors that would cause older surveys not to line up correctly, I am wondering if the way surveyors use to measure the terrain and elevation would account for some of these discrepancies.
Older surveyors all attempted to measure flat, and we still do today. They measured large parcels of ground with a steel tape (or before that a metal chain). There was plenty of room for error. But also note that we all consider original surveys to be "without error" so it's our mission to find their original marks in the ground. The older surveys established their basis of bearings in a more archaic fashion as well. And, again, We do our best to find their original marks and adjust our surveys to theirs.
But answer to your question, they did not measure "slope distances".
The unfortunate thing is that a huge percentage of descriptions were first written by non-surveyors. The landowners themselves or some "brain" like an attorney, appraiser, real estate agent, elected official or bureaucrat may have put the first words on paper. Sometimes those words were based on measurements or approximations provided by someone else using whatever method they had available to them. Pacing was common. Later, stadia shots may have been used. Measuring on the slope by non-surveyors would be a common dilemma. I'm aware of a case from the 1860's where one of the participants in laying out the little town reported decades later that a 25-foot stick and a grapevine were used to establish the measurements placed on the plat. Not sure how one standardizes the length of a grapevine.
Tom and Mr. Cow are both correct. Maybe I can restate and make it more clear.
Elevations have "always" been considered. Distances are all intended to be like it is flat ground.
Ortho Photos can be accurate, but depending on how they are created they can be very crude... it depends on the camera quality, elevation of the camera, and what objects on the ground are used to scale the photos.
Some Ortho Photos are "accurate" plus or minus 1 or 2 feet, many only plus or minus 10 feet. It depends greatly on the way the photos were taken and how they were compiled.
I caution you on where you "think" those objects on the ground that you see in the photos. Many times fences and roads were Not constructed on the property lines... sometimes by accident, sometimes on purpose.
Technology changes, accuracy requirements change.
Modern surveyors constantly struggle with crude GIS systems and tax maps that have exaggerated the inaccuracy of the actual ground measurements.
20' in elevation change = 1ppm for distance
200'=10ppm
so your elevation factor shouldn't really impact putting in GIS info which is usually "reduced" to state plane anyway, these are very minor adjustments compared to the much larger issues regarding errors found in the cadastre.
I would think other factors swamp elevation "error" very quickly in your area.
Thanks for the replies, very interesting information. As far as the tax maps I am working on, I'm trying to get them as accurate as possible, but given some of the vague deed descriptions, it's impossible. Unless there is an actual plat recorded showing the roads, etc, then sometimes all I have to work with is "Beginning at a big rock thence 30 rods in an easterly direction along the dirt road to an oak stump". I pretty much have to make the assumption that the paved road is the same as the old dirt road mentioned in the deed 80 years ago. I think I would go crazy having to do new surveys based on some of the deed descriptions I read. This is for a very rural area, so I doubt they had many professionals doing the surveying originally.
More than likely that road is not where it was 80 years ago.
Usgs earth explorer may have some photos from way back that might be helpful to you.
Jambog82, post: 344913, member: 10787 wrote: Thanks for the replies, very interesting information. As far as the tax maps I am working on, I'm trying to get them as accurate as possible, but given some of the vague deed descriptions, it's impossible. Unless there is an actual plat recorded showing the roads, etc, then sometimes all I have to work with is "Beginning at a big rock thence 30 rods in an easterly direction along the dirt road to an oak stump". I pretty much have to make the assumption that the paved road is the same as the old dirt road mentioned in the deed 80 years ago. I think I would go crazy having to do new surveys based on some of the deed descriptions I read. This is for a very rural area, so I doubt they had many professionals doing the surveying originally.
Yes, some of the old descriptions are pretty difficult to decipher, but that's just part of the challenge. All surveyors understand that tax maps are simply drawn to the best information available. The difficult part is getting the public to understand that.
I have been told that there were some cases where slope distance was used in deed descriptions, particularly in mountainous areas, but I can't give any specific examples and it would be very rare. Perhaps some others on this board have more information about this.
"I pretty much have to make the assumption ..."
Making any assumptions whatsoever with TAX MAPS is a truly frightening thought. If the property valuation is area based you are setting your employer up for a HUGE issue!
Your real problem is that West Virginia is not flat. Many early surveys were actually measured and record on the slope.
A modern survey is intended to be on a flat surface, however the distances may vary from the lowest to the highest elevations. A meaned elevation used for Parcel A may not be applicable to a meaned elevation for any adjoining parcels. Orthophotos and tax reference information should be on the SPC ellipsoid. You may need to convert modern field survey information to SPC ellipsoid. For your purposes USGS to Geoid to Ellipsoid should get you in the game.
With record information you may required to guess how it may have been originally done.
BTW what are you educated in?
Paul in PA
Western North Carolina is the about the same as West Virginia and if you get behind an old survey described in poles, most of the time you will find the monument at a shorter distance than is on the plat, due to the slope. I am not old enough to have experienced it first hand, but I have been told a lot of the time they just kept flippin the pole end over end.
I grew up in WV and the old timer non-surveyors would always talk about "measuring on the ground surface" when they discovered that I had become a surveyor. I've never done a survey in WV, but from these discussions I am of the opinion that very old surveys may have been done on the surface of the ground by "surveyors" who may not have had the expertise to be surveyors at a later time in history.
Vern, I was borned in West By God Virginia too! In Raliegh County, My old man was a coal miner and he headed south on the hillbilly highway in 1987 to find work. Where about's you from?
What????? If you have 20' elevation change in a 20' distance, you'll have a lot higher ppm than that. I'm not sure what you're talking about (though I'm sure it has something to do with GPS).
I know you're a lot smarter than I am, so I'm sure referring to something different than what I understand the topic to be.
Jambog82, post: 344913, member: 10787 wrote: Thanks for the replies, very interesting information. As far as the tax maps I am working on, I'm trying to get them as accurate as possible, but given some of the vague deed descriptions, it's impossible. Unless there is an actual plat recorded showing the roads, etc, then sometimes all I have to work with is "Beginning at a big rock thence 30 rods in an easterly direction along the dirt road to an oak stump". I pretty much have to make the assumption that the paved road is the same as the old dirt road mentioned in the deed 80 years ago. I think I would go crazy having to do new surveys based on some of the deed descriptions I read. This is for a very rural area, so I doubt they had many professionals doing the surveying originally.
I'm glad you came here for advice. I think the take-a-way is that you don't go thinking that you have some kind of model than what a land survey would create. GIS is good, but some GIS Specialists often think that their knowledge is superior to a land surveyor's. It's not an intelligence contest, it takes real property line retracement to understand the imprecisions and what a property looks like on the ground.....just like many of us wouldn't be able to create a GIS model.
I was born in Elkins grew up in the Mill Creek and Valley Bend area, went to Tygarts Valley HS
Jambog82, post: 344883, member: 10787 wrote: When surveys are done, is it standard or even required to take elevation into account? For example, if you take two points on a slope, the distance between the two points on the surface would not be the same if measured on an ortho photo, correct? So do modern surveys account for elevation so that when laid out over a flat ortho photo, it will appear more accurate? If so, has this always been standard surveying practice, or was there a time when elevation wasnÛªt accounted for and only the distance between the two surface points were measured. When did this become standard or even required practice for licensed surveyors?
The reason I ask is I recently started a job working on tax maps. This is my first job doing anything with mapping, and I come across many older surveys from decades ago that donÛªt fit very well over the photos and I have to make a lot of adjustments to fit within the roads, fence lines, rivers etc, that they should be lining up with. While IÛªm sure there are many factors that would cause older surveys not to line up correctly, I am wondering if the way surveyors use to measure the terrain and elevation would account for some of these discrepancies.
Build your gis from the known to the unknown. Input the road network. Input the subdivisions and more recent surveys. Isolate the vague areas. Chip away.
You will still have problems.
For example. I have a 1927 state highway layout that would be a great backbone. The math is perfect but the monuments do not work. As we survey that stretch of road, we break the layout and hold the monuments where we can get agreement. We hope in a few years we will have it all resolved without having to sit down and do it all at once...
Are these owner known properties?
I think I read once in Browns Evidence and Procedures for Boundary Locations that many of the older surveys in colonial states would have their distances measured along the ground. These dimensions would have made there way into record descriptions, so that may be what you are seeing. Also keep in mind your orthophoto is probably on a grid coordinate system that has scaling factors to covert between ground and grid dimensions (and in mountainous areas these scaling factors can be quite significant).
What really makes a mess is when another retracing surveyor sets an iron at the deed distance of exactly 100 poles in mountain land, crossing a fence, the called for monument, another fence and ending up a couple hundred feet into the adjoiners pasture, in the process.
