SPC question or help

> "The coordinates shown hereon are Texas South Central Zone 4204 State Plane Grid
> Coordinates (NAD83) and may be brought to surface by applying the following combined
> [scale fact]or 0.99998984736"

Ground Distance x Scale Factor = Grid Distance

Grid Distance / Scale Factor = Ground Distance

So if you take all the Grid Coordinates and divide them by the combined scale factor the inverses you get using the resultant coordinates will be the horizontal distances you would measure at ground level (as opposed to distances along the grid projection).

> I'm not sure I even get this. I have a plat with:
>
> "The coordinates shown hereon are Texas South Central Zone 4204 State Plane Grid Coordinates (NAD83) and may be brought to surface by applying the following combined
> or 0.99998984736"
>
> N: 13861377.566 E: 3114365.827
>
>
> I would like to ask for some help in understanding how this works and roughly 100'+/- where this would plot on say Google Maps?

Taken at face value, the plat is saying that the coordinates on it refer to the Texas Coordinate System of 1983 (South Central Zone). So, if you run the coordinates as supplied through Corpscon, those should yield the NAD83 latitudes and longitudes of the points positioned at whatever epoch the surveyor of the plat used in his or her survey.

The "brought to surface" is just superfluous BS unless the plat is annotated with surface distances. In that case, you multiple the plat distances by the combined scale factor to get the corresponding grid distances.

Kent & Norman, Thank you. I had run these coordinates through Corpscon and for a quick look plugged my traverse and the plat coordinates into Google Maps. The traverse points plot as correctly as Google Maps will allow you to zoom but, when I plug in these plat coordinates it plots in the most ridiculous place, and the inverses between the lot corners and traverse are out by roughly one quarter mile.

Kent, what measure of difference would I expect between epochs of roughly five years?

Thanks for the help on this.

> Kent, what measure of difference would I expect between epochs of roughly five years?

In Texas, I would think the changes from epoch to epoch of the NAD83(CORS96) coordinates of some point wil be quite small (sub-meter for sure and probably sub-decimeter), even from Epoch 1993.0 to Epoch 2010.0. You can test this by examining the data sheet of the nearest control point in the NGS data base with coordinates published for various epochs. If the coordinates don't plot basically on the ground feature in Terraserver or Google Earth, the most likely hypothesis is that the coordinates are grossly erroneous. Most likely they are bastardized coordinates of some sort.

> In Texas, I would think the changes from epoch to epoch of the NAD83(CORS96) coordinates of some point wil be quite small (sub-meter for sure and probably sub-decimeter), even from Epoch 1993.0 to Epoch 2010.0.
My experience is similar in both Oregon and Oklahoma. '83(2011) changed coordinates from the previous epoch on the order of a centimeter or so.

This doesn't get any better on the ground, and now I understand why there was a request for a new survey, why the current landowner was bird dogging me as to whether I had found any discrepencies that would hold up the sale, why the occupation is a foot and a half short and the block corners a couple feet long. There's a whole row of these cheezy little new homes built by the same builder, and of course they are exactly on the building line according to the platting surveyor's lot survey.

What a bad scene. I had hoped the SPCs would shed some light on this.
Thank you for the help on this.

> What a bad scene. I had hoped the SPCs would shed some light on this.

Well, the coordinates in the Texas Coordinate System of 1983 actually do shed light on the situation. If a surveyor is reporting coordinates as referring to that system and they obviously don't, it immediately says that the effort is substandard in at least one way and likely a number of others. That's the great thing about the SPCS: you can only fake it so far and then it's obvious that you're faking it.

First Step Is Bring All SPC Coordinates Into A CAD Program

Then you scale those coordinates around N: 13861377.566 E: 3114365.827

N: 13861377.566 E: 3114365.827 = N: 13861377.566 E: 3114365.827 before and after the scaling.

Sounds like you were scaling around 0,0, that should solve your distance problems.

Paul in PA

R. J., the first thing to do

is realize that Google Earth is already in grid. You need to convert the State Plane coordinates to Latitude and Longitude. No surface conversion is needed.

R. J., the first thing to do

Appreciate the help on this. I really didn't have any glitches plotting the traverse to Google. Guess I just flipped out when i couldn't get any two things to work together.

Thank you again registereds.

R. J., the first thing to do

When reviewing subdivision plats, that's (SPC's) probably one of the top two errors that I see. Surveyors claiming that they're reporting SPC's, when they're actually reporting some bastardization that's been scaled to surface.

On plats or engineering plan set reviews, they rarely supply a metadata statement. After being told it's required, they still believe that everyone knows the coordinates (i.e.- 0,0) of their scaling point. Still yet, they find it hard to believe that someone might scale the coordinates to the surface from a point central to the project. In other words, they can't be bothered to actually tell folks what they've actually done in a metadata statement.

At the end of the day, I usually have to hand hold with their technician, engineer, or themselves, and explain what real grid coordinates are. The majority are under the mistaken belief that surface coordinates are REALLY grid coordinates.

R. J., the first thing to do

> When reviewing subdivision plats, that's (SPC's) probably one of the top two errors that I see. Surveyors claiming that they're reporting SPC's, when they're actually reporting some bastardization that's been scaled to surface.

Yeah. That's exactly what leads me to conclude that the use of the Custom Projections will only make things completely SNAFU. I mean, if you can't use a legally defined, well documented projection like the SPCS that is also well supported by a whole bunch of software used by surveyors, what chance is there of the product being even recognizable as the work of human intelligence if the projection is a one-off version with unique parameters to be arrived at by the same technicians who didn't savvy the SPCS in the first place? I'm thinking the odds are pretty dang small for that being a success story of any kind.