Georeferenced Control Points/World File

[Craig_Eissler_Iceman]

When Georeferencing (DWG CAD data) limited to only 2 Control Points, the alignment is OK, but not great. It automatically produces a World File that I show below.

1. Do the results look odd? They sure don't resemble the ArcGIS Pro Help doc example.

2. And, where values do exist, they do not appear to have anything to do with the Coordinate System ...why? 

3. Finally, what do each of these 6 Columns represent? ...The Help doc was confusing to me.

Thanks, CCE

[ThomasHoman]

Looking at https://pro.arcgis.com/en/pro-app/latest/help/data/cad/world-files-for-cad-data.htm 

• Line1 - <from-pt1x,from-pt1y,from-pt1z> <space> <to-pt1x,to-pt1y,to-pt1z>
• Line2 - <from-pt2x,from-pt2y,from-pt2z> <space> <to-pt2x,to-pt2y,to-pt2z>

where:

• from-pt2x - starting x (E) point in cad dwg
• from-pt2y - starting y (N) point in cad dwg
• from-pt2z - starting z point in cad dwg
• <space>
• to-pt2x - destination x (E) point in cad dwg
• to-pt2y - destination y (N) point in cad dwg
• to-pt2z - destination z point in cad dwg

They looks perfectly fine compared to the help example, it's just that your cad dwg started from a different location thus the zeros on the front end and since there was no vertical transformation the z values are zero. 

With this information, it appears that the cad drawing was somewhere around 0,0,0 and moved/transformed into the neighborhood of 1240814, 375667, 0 (X,Y,Z)

Something to consider, in most coordinate systems Y equates to north and X equates to east, so does reversing the coordinate make sense for your particular coordinate system 375667, 1240814?

Regards,

Tom

[Craig_Eissler_Iceman]

Thanks, Tom.

Here's a re-do. The Map View and Reference (Target) data are set to the same projected coordinate system.
The CAD has an Unknown coordinate system.

The Control Point table definitely reflects those projected coordinates, but I still cannot make sense of the coordinates in the associated and, automatically created, World file. ..Any clue?

Also: I know what a PRJ file is, and noticed that was automatically created as well, and was assigned the aforementioned projected coordinate system. ...So, does it just take this from the Map coordinate system?

By the way: I also tried assigning a coordinate system FIRST to the CAD DWG data to see if that changed anything. The Control Point table looked the same but the World file had completely different values, and still not the real projected coordinates.

[ThomasHoman]

Hi Craig,

Sorry for the late response, it took some digging to remember where I had seen the answer. The .wld file is  the information necessary to implement a linear affine process "to move scale and rotate the CAD data from where it was drawn to the proper coordinates..."

https://pro.arcgis.com/en/pro-app/latest/help/data/cad/about-cad-coordinate-systems.htm#ESRI_SECTION...  

Hope this helps

Tom

[Craig_Eissler_Iceman]

Thanks, and yes, I understand that concept. With that, I am trying to relate the World File to the Control Points table after Georeferencing the CAD Data in my screenshot. I totally understand the Control Points because these are the geospatial coordinates for the projected coordinate system I'm using. But, I still don't understand what coordinate system the World File values are referencing (especially columns 4 and 5) because I've never seen those values before in GIS. ...If they're just CAD Cartesian coordinates referencing 0,0 somewhere, then I guess I'm wondering what "units" are these in? Another way of asking would be "if I was creating this World File myself (to load into the Georeferencing tool) how would I know what values to use for any of the inputs, including columns 4 and 5?"

[ThomasHoman]

The value represented in the World File only represent the 2 vectors necessary for the linear affine math to do it's job within the currently assigned map projection. This is why the .wld file is only vaild for a single projection - change the projection and .wld breaks.

For your last part to precalulate the .wld file values, I can only say this is way outside my mathematical ability to answer. OSGEO has a discussion of the linear algebra steps that may help your inquiry. https://trac.osgeo.org/postgis/wiki/DevWikiAffineParameters