topic Re: Coordintate transformation issue in Civil 3D Forum

Coordintate transformation issue

Anonymous — Mon, 20 Mar 2017 20:25:23 GMT

Greetings, i am trying to tranformate a .txt grid coordinates in "Regven" reference datum to "La Canoa" reference datum. Both systems are in UTM projection. Both datums are used in Venezuela, South America.

I made a point file format that has regven as the coordinate system

And i have selected the other datum called "La Canoa" in the drawing settings.

The issue is that when i import the txt point file, the software calculates a grid norhting and easting value that differs 7 meters from one i calculate with another verified software.

Also, when I decide to use another "La Canoa" datum from the list that i can see in the drawing settings, the values calculated are even more different.

That other "La Canoa" datum gives me even more meters of difference with my reference value obtained from another software and it should be the same, at least the same from the first one i choosed, because theres only one "La Canoa" datum in venezuela for zone 19.

Am i doing something wrong or the softwares it not calculating this correctly? Thanks in advance.

Re: Coordintate transformation issue

Anonymous — Mon, 20 Mar 2017 22:10:06 GMT

Have you tried any other approach to the transformation? I haven't used 2017 much, and can't address what might be right or wrong about your current workflow; but I have had trouble trying to import and transform survey data in a single step. I get the most consistent results using the workflow recommended in this blog: thecadmasters.com - transformations-part-2-re-projecting-drawings/

You might also try QGIS - it's a free GIS platform that makes transforms pretty easy, although it doesn't always produce identical results to civil3d.

Re: Coordintate transformation issue

Anonymous — Mon, 20 Mar 2017 23:37:15 GMT

The workflow/method i am using is detailed in the softwares help and in the mastering civil 3d book series. What gets me is that the obtained values doest not match to other software conversion and if i decide to do it using another equivalent datums, it shows different values. Regven is also WGS84 datum and La Canoa is also PSAD56 datum.

I could use another software but that is not the point. I am doing this because i want to prove and verify civil 3d coordinate transformation precision.

Re: Coordintate transformation issue

Anonymous — Tue, 21 Mar 2017 06:21:59 GMT

What is the transformation method that the software uses? I am comparing my results with a molodensky-badekas method. But still, 7 meters of difference is a lot of error and it does not justify that i get a very different result if i decide to use the other equivalent datums on the list.

Re: Coordintate transformation issue

Pointdump — Tue, 21 Mar 2017 10:34:34 GMT

"...differs 7 meters from one i calculate with another verified software. "

What verified software? Have you checked the Parameters for mistakes in _MAPCSLIBRARY? For instance, is the Central Meridian's Longitude based on "0" or "Greenwich"? Small differences abound. And I like @Anonymous 's idea of checking with QGIS. QGIS has very up-to-date coordinate systems.

Another thing to check, is the "Local" Northing and Easting the same as "Grid" Northing and Easting on the Transformation Tab? Sometimes if you Copy/Paste to make them match it solves little problems like this.

Dave

Re: Coordintate transformation issue

Anonymous — Tue, 21 Mar 2017 16:33:57 GMT

The transformaition tab settings are off because i am not converting grid coordinates to terrain or backwards, i am just transforming a datums grid coordinates to another datums grid coordinates. (Using point file format with a different datum than the drawing settings datum). In other words, theres no need to use scale factors for this kind of transformation.

The software i am using was developed by a goverment institution in my country that only does the transformation REGVEN-LA CANOA. It uses the molodensky-bedokas method.

What i wanted to know is what method using civil 3d to determine the coordinates value. 7 meters is a lot to me, and i dont know if this is because of the method or because theres something wrong with civil 3d there.

I am guessing the problem is civil 3d because i tried to convert to equivalent datums and the results were pretty different too, at least they should have been the same for the 1st try.

Re: Coordintate transformation issue

Anonymous — Tue, 21 Mar 2017 23:11:16 GMT

Alright guys! I did two things:

1- I went to MAPLIBRARY to see the parameters of REGVEN and LA CANOA equivalent datums and found out that some had a false easting that explained why sometimes the software gave a different result compared to the 1st one.

2- I managed to learn to use QGIS for the transformation, and he values i got were very similar to Civil 3D. This is very important because now i have to find out what the hell is the software from my country doing, i still dont know if this is because the methods are very different or the software has something wrong (a fact that could be very disturbing because 99% of the people in my country use it for the transformations).

I just wish I could know what transformation method is using civil 3d and QGIS to determine the values.