Conversion from NAD27 to NAD83

You might try Corpscon6, should get you close enough to find them.

James,

I think Corpscon 6 will do what you want. Get it here:
http://www.agc.army.mil/Missions/Corpscon.aspx

I've downloaded it, but haven't used it yet. Lots of really saavy C3D users swear by it.

Dave

corpscon has always worked for me- cept now i can't get it to open in up wonderful windows ocho.

so i've done a few with the roundabout way- excel and the NOAA nadcon converter utilities.

basically NAD27>NAD27 lat and long>nad 83 lat and long>nad 83.

Please be aware that there is no accurate way to convert between nad27 and nad83. There's software mentioned here, corpscon, but don't assume the conversion will be accurate.

The conversion can be inaccurate from a few feet to tens of feet.

The only real way to accurately convert between nad27 and nad83 is to create your own local adjustment by occupying nad27 control points on the outer edge of your project with your gps and performing an adjustment.

> Hello All,
>
> I am in the middle of a project where I have an old hand drawn map with coordinates in NAD27. I have my project in NAD88 via GPS. This old acquisition map has called for monuments that I have to retrieve but they are in marsh. Ultimately I would like to be able to stake out to the control so I am not wondering threw the marsh and reeds hoping to stumble upon it.
>
> I am currently running Carlson 2014 on Autocad c3d 2014. Is there a way to enter the NAD27 coordinates and have them spit out NAD88 coordinates? Is there a converter somewhere on the internet? This is a new area for me and no one else in my office has experience either.
>
> Thanks for your help
> James

While frequently not acknowledged, the USACE tool CORPSCON is an interface for an NGS suite of tools. Performing NAD27 to NAD83 transformations is done via NADCON. An article describing its use and some details is here: http://geodesyattamucc.pbworks.com/w/file/53304733/NADCON_ProfSurv_article.pdf

This is a pre-print.

Nadcon but be careful as it isn't an exact transformation. A couple of weeks back I was working with mixed control and for that area the Nadcon conversion was off 1.5' from observed values.

I can't agree with AL more. There can be some large differences between the conversion and what is on the ground. And be careful that the 27 is actually 27 and not a modified 27. That being said, you will probably be within two feet or so if the 27 is 27 and it was well done.

You didn't mention what kind of GPS you are using, if it's Trimble then the conversion is simple, Trimble will automatically convert for you and have both sets of values in the job, nNAD83 numbers and 27 values both imbedded in the file..

Read the documentation linked by GeeOddMike. If you're using CORPSCON/NADCON the 2 sigman (95%) confidence for the transformation from NAD 27 to NAD 83 (1986) is about .3 m (1 ft). It is generally much better. On the numerous occasions where I was presented with shifts by surveyors that were significantly larger there was always a good reason, such as the NAD 27 values they were comparing were either from an unknown source or from surveys tied to NAD 27 control that was poorly tied to the network such as USGS TT stations or Corps of Engineers. As noted by others there is no such thing as a transformation that will absolutely maintain the integrity of a high accuracy NAD 83 survey going back to NAD 27. Also remember that CORPSCON/NADCON will only go from NAD 27 to/from NAD 83 (1986) to/from NAD 83 (HARN). If you have data in NAD 83 (2007) and/or NAD 83 (2011) you need to use GEOCON and GEOCON11 as well.

I don't doubt you're right. The NAD 27 values on the project I'm speaking of come from DOT monuments and the DOT didn't document the source ties that created those values. Very much likely those old surveys had that amount of slop.

thanks

Thanks for the awesome feedback gents. I am using a Lieca ATX12300GG gps. I would be happy to be withing 2-3 feet on this project. I am going to leave the information in 83. The old baseline is along the shore line, and there has been plenty of accretion and there is quite a bit of high reeds. I am looking to go armed with the old data so I can be in the right ball park when looking for the needle in a haystack.

Thanks again for the info!

James

Generally I've been within a foot or so on 1st order NAD27 monuments near HARN points, however, I've seen almost 6ft on a fist order monument far removed from the HARN network. And on 2nd and 3rd order monuments the values are hit and miss. As far as control added by DOT or private surveyors I don't see it any worse or better than the government points.

MightyMoe could you please post the PID of the station you missed by almost 6 feet, I'd like to take a look at that. Please also include the NAD 83 position you determined for it.

:good:

There is a tool for the conversion between 27 and 83 in your Carlson 2014.

Under the Points pull-down menu, there is a Coordinate Transformation menu item.

Clicking on this will bring up a dialog box that offers several options, among which is a grid-to-grid conversion.

The warnings about the possible imprecision of such a conversion notwithstanding, this utility should give you the search points you are looking for pretty easily.