My water guy is finishing a plat for a water plan. I'm glad I'm not the one doing it, it is confusing to say the least, pouring over all the varies rights, petitions, consents, changes......it gives me a headache.
So we have a few simple bearing and distance ties on the map, which means I want a metadata statement.
Usually it includes a DAF (Datum Adjustment Factor) so someone can recreate the distance from the state coordinate system. This time we are tied into a DOT project that set control along a nearby highway and we later used it to do ROW plats for them. Simple
The DAF is 1.00024419, a typical DAF from the DOT for a 10 mile or so highway project.
So my water guy asked why it's so long since we usually use one that is six places.
I told him there is no good reason to make it past six places and then he said maybe the DOT wants it as accurate as possible so I made a chart.......
Using the very rough number of 20,000,000 ft for the earth radius and only considering that as the controlling factor, since the easting factor is not very important across a small slice of eastings....... I made this crude chart.
So why are the DAF's for the highway projects so long......hate to say it but engineers.........;-)
Repeatability is a more likely answer. Many coordinate systems result in northing or easting ordinates in the millions. When you have software packages requiring CAF on one end and 1/CAF on the other things get sloppy real quick without the 'extra' digits...
The DAF is just a number that is assigned to each project. They try to simulate the surface distance by multiplying the State Coordinate by the DAF, then when the coordinates are inversed the distance should match the surface number very closely.
In this area there was a historical one (NAD27) that was used for a large area- 1.0003. When GPS came out they could see it was a bit crude so they started to do 10 mile projects with these long ones, going overboard in the opposite direction.
But it's all good, whatever one gets assigned I use. The last control project I did for them, I used a five place DAF since there was 200' of elevation difference from north to south. Going to 6 places on that one didn't accomplish anything.
Exactly. And to help with that, do not calculate and use the reciprocal. Rather, multiply and divide by whatever scale factor you are using.
The work flow in many outfits (mine included) uses a predetermined CAF for various geographic areas. Keeping them consistent allows smooth interchange of data with little or no effort. Unfortunately code writers being the contrary folks they are, some packages require CAF and others 1/CAF. When you switch up a coordinate with a high value things get moved around. In modern software (think parcels, polygons and alignments) a thousandth of a foot blows up as bad as a foot.
For the Chief in the field doing a standalone lot survey it doesn't make a difference. That is especially true with simultaneous creations where no map will be published. I haven't done one of those in over 15 years....
Exactly how I work. The projection is setup and entered into the computer and DC before going to the field.
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> So why are the DAF's for the highway projects so long......hate to say it but engineers.........;-)
- ummm, if only that were the case.......
