Drain field design and "As Built" drawings

I recently requested an "As built" drawing from my county health department for the septic system. What they faxed back to me was not exactly what I was expecting to see. It was dated 1960 and listed the owners, builders, and inspector.
I happen to have a degree in engineering and I was expecting to see an engineering drawing or something official (maybe even on vellum) - what I got was best described as "back of the napkin with a coffee ring as a bonus decoration". First, is that realistic, I don't want to harp on it if in fact that's what they're all like, and second how truthfully can I take the measurements and layout of the "drawing"?
I'm looking at the drawing and it looks like they deliberately routed the drain field so that it goes off in weird angles, makes several beelines for the massive Douglas Fir in my backyard and otherwise doesn't efficiently utilize the space provided for it. I was expecting a nice layed out route involving 90 deg bends and long branches. What I see is the first line making a large oblique angle into the middle of the yard then sending branches back toward the tree, finally connecting back to itself after circling the tree.
Was it common to design the field to incorporate trees as a means of absorbing water?
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Eigenvector wrote:

You should consider yourself lucky that the building department found a 45 year old drawing. In many parts of the country, no records were made or kept accessible. TB
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i don't know, but you're lucker than i am. some government employees were told to clean out an old room and threw away filed and approved building plans in an old storage room in buffalo ny city hall back in the 1960s including some of mine from the 1910 era. in your case hidden underground rocks and the number of years that have gone by are other factors besides lost memories of former owners as to where all the pipes are. your thirsty tree probably has also enjoyed free fertilizer from your system for many years.
Eigenvector wrote:

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I'd guess that probably 80% of the homes standing today were built from plans with three sheets: Elevation, basement/foundation and floor plan. Most lumber yards had draftsmen who would draw up house plans for free if you bought your lumber from them. Until lately, only big cities had inspectors. My own home town finally got an inspector in the late 1960's, and he was (theoretically) responsible for everything including plumbing connections to the water and sewer, septic, electrical and finished.
Outside of big cities, which had subcontracting, most small-town generals had crews that set the footers and basement forms, framed and did everything. Through the 60's, my Dad had a 6-man crew that built darned good houses and the only subs were for excavation, plumbing, HVAC and drywall. I did most of his electrical, we all pitched in on the drywall and flatwork, painting, roofing etc. Besides being a darned good education for a teenager, it also produced a very responsibly constructed house.
If a house had a septic system, the old boy with the backhoe was the one who laid it out from the seat of his backhoe. IMHO, most of those old systems are probably as good as the engineered ones, but if something DID go bad, it really went bad,
Nonnymus
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Aren't trees bad for the drain field though? It is conceivable that the tree grew after/at the time the house was planted, maybe even the construction helped bury the seed that spawned the tree?? I don't know, but it's definitely around 40 years old, its a BIG tree but then again Douglas Firs grow pretty fast. I ought to scan in the "As built" to show you how primitive it is, although if the responses here are any indication its extremely well crafted by comparison - heck it's on graph paper! Still, I have to wonder about the crazy angles and poor overall layout of the drain field.
If/When the city does finally incorporate my development and routes sewer through the neighborhood, do I have to do something to deactivate the septic system? Can I just dispose of the tank, re-route the sewer line, then let the trees and bushes take care of the drain field?

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The "as built" drawing sounds at least as good as the one I got, from a system put in not 5 years ago by the previous owner. It is a piece of letter-size paper, with some blanks at the top (address, date, contractor, etc.), a few signatures at the bottom, and a hand-drawn figure in the middle. At least they used a ruler.
It isn't to scale, the angles are all just sort of approximations, the distances are completely wrong, the landmarks they picked to do a triangulation no longer exist (a fence post and a bush? come on! both corners of the house are right nearby and could have been used easily). Not to mention that the triangle is so acute that even if the distances were correct, the +/- a few inches makes the location swing about +/- 10 feet.
I ended up just doing some digging, some poking around, and found most of the lines. I then took some pictures from an upstairs window, and overlaid lines on the (digital) picture with measurements. (As for poking around, if your's is very shallow, like mine... I took a piece of 4-foot rebar and went out after some very heavy rain. I was able to poke it in by hand about 2 feet, deep enough to feel the gravel in the drain field, and was lucky enough to hit and locate a few pipes that way too.)
Eigenvector wrote:

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