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Workflow from geographical coordinates to "CAD"


#1

As the title suggests.

Once obtained the 3 geographical coordinates (latitude, longitude and ellipsoidal height) how to transform them into X, Y and Z (geoidic height) ?

QGIS ? How to do it automatically for many points ?

Is there a tutorial ?


#2

While I haven’t used the tool yet, found this in the NOAA website;

https://vdatum.noaa.gov/

Not sure if this will work for you, but worth a shot.


#3

One could load grid geoid file with surveyed points into QGIS project. Run in SAGA toolbox add grid values to point. Add additional field in your point dataset to calculate orthometric height as you have both the ellipsoid height and geoid height as part of point values. H=h-n where H is orthometric height and h is your ellipsoid and n being the geoid value.


#4

This would be a great solution for my needs.

Unfortunately I don’t know the QGIS software very well, is there a guide or a video on youtube that explains step by step how to do it ?

Thank you.


#5

How many points? I would you use the above link if not too many.

I like vdatum and use the application you can download to computer. You can batch by processing your survey file to a text file. See here:

https://vdatum.noaa.gov/docs/userguide.html#step31


(Timd1971) #6

Or use MicroSurvey FieldGenius 10 on a data collector or Surface or rugged Windows 10 tablet (amazon or google keywords: gnss tablet)

Then it works hand in hand with MicroSurvey CAD 2019 which works like AutoCAD (Civil/Map). It has all your coordinate systems and can load geoids. No i dont work for MicroSurvey, its just they have got the data collector software nailed down. Where ReachView Survey is in its infancy compared to it.

Get a RS2 (multi-freq) so you don’t have to waste your time with the RS/RS+ (L1 only). Might as well get a loan for a Leica robotic total station while your at it.

Money money money MONEY!!! $$$$$

; )


#7

I’m very confused and I need a cheap solution.

Using Reach and RTKLIB (if PPK mode) or Reach and ReachView (if RTK mode) I get the geographic coordinates of all survey points.

What I want is to load these points in QGIS and export them as X/Y/Z .dxf CAD file. The Z should be geoidic.


(Michael Lambert) #8

Export the CSV. Import into QGIS as a delimited layer and save back out as a transformed CSV. Import into CAD.


(Dmitriy Ershov) #9

Also check this out:


#10

Sorry but something still does not work :frowning:

Thats my workflow:

  1. I open QGIS and load the Google Satellite layer (EPSG 3857)
    https://mt1.google.com/vt/lyrs=s&x={x}&y={y}&z={z}

  2. I project this layer in the EPSG 4326 system (WGS84).

  3. I add the CSV file containing the geographical coordinates measured by the GPS (WGS84).

Problems:

  1. The measured points do not match the satellite image. There is an error of a few tens of centimeters.

  2. QGIS does not perform any processing on the ellipsoidal Z dimension that remains ellossoidic.


#11

One thing you can always count on is satellite imagery not lining up with reality. Most of it is close or close-ish, or not very close :slight_smile:

But with QGIS, you have the option of taking some imagery and aligning it correctly (geo-referencing).


#12

If with the same measured points I generate a .kml file and load it on Google Earth the correspondence is perfect. Is there something wrong with the process with QGIS…


#13

I don’t know, but maybe someone else will have a suggestion for you.


#14

There is nothing wrong with QGIS, just a few more suggestions. Take CSV from reachview import into Excel. Modify that CSV to match the formatting needed for vdatum (in link I provided). Save as delimted text file. Batch process in vdatum and then import into QGIS as a delimeted layer defining exactly the coordinate system you need. Once loaded right click your imported layer in layer tree and save as DXF. It is now a CAD file in your coordinate system of choice.

The horizontal coordinate system is not necessarily tied to the vertical coordinate system. You would have converted XY but it does not transform the Z. You need internal geoid file for that so please try as we suggest above and see if that works.


(Michael Lambert) #15

I have experienced the same thing. It is the display of the points, but they are correct. If you import points or draw lines and zoom in and out they will appear to go off line until you right-click and zoom to the layer. Kind of like REGEN in CAD. Google Earth does display better.


#16

I downloaded vdatum.

It looks very “US oriented”… does it work for the European region too? I see many acronyms I don’t know…


#17

Ok, let’s take a step back. What coordinate system and geoid model do you need to convert to?


#18

Reach returns points in terms of geographical coordinates (WGS 84). I’m interested in taking these points and projecting them into a local cartesian coordinate system (Eulerian system).

The origin of the CAD reference system could be a point within the coordinate 0,0,Z.

In other words I’m interested to put all these geographical positions into a simple .dxf file in which I can read and draw distances and height differences beetween all these points.


#19

What are your local coordinate system definitions and is it tied to local benchmark (XY)? What geoid model for Z?

You can send me your data privately and I will attempt to output into your desired coordinate system as .dxf if have time. Then I can put together tutorial with visuals and my rad paint skills though not as refined nor am I certified as others on this board like @bide :joy:
image


#20

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