georeference north

[Benson Shaw]

Trying to understand North in georef context.  In the example image, the file has a US Washington state plane CRS as noted.  County GIS data portal provides the section and quarter section geometry and data.  VWX internal origin and user origins are moved to NW corner of the section of interest.  This all lines up nicely with the ESRI geoimage and street map.  Some comments:

1. TopPlan xy geometry, eg the property line, shows north always parallel to the y axis.  I think that's good. Or should it lean?
2. Section lines in USA can adjust to local topography, but idealized version is a square mile with edges on NS and EW.  The quarter sections from this township do not georef closely to north.  
3. Noted that public maintained GIS data is not presented as highly precise. It's not same as commissioned property survey.  The data portals all have warnings that it is more for cartography and not basis  for legal or real estate transactions.  So, grain of salt on these geometries.
4. Great circle north varies about a degree from y axis at the internal origin. Is there somewhere in the drawing where great circle north is same direction as y axis?  eg at the CRS meridian? how to find that?
5. Relocating a great circle  east/west (eg drag or move by points) changes its bearing.  That makes sense, because the bearing must adjust to maintain the connection to the polar rotation points.

So, a few questions:

• In a survey withp roperty boundary bearings and distances, and CRS/projection notation, is vectorworks xy geometry appropriate, eg the prop line with its bearing notations?
• What conditions would cause a need to adjust the georef setup angle to true north?
• Or is this north rotation option sort of a georef version of drawing plan rotation - used for graphic or workflow convenience?
• I would like to see the great circle tool have editable bearing/distance values.

 

OK, just trying to understand more of the georef features.  Thanks for any comment or experience.

 

-B

 

[Benson Shaw]

Or, another way to ask —-

The sample shows 3 different approaches to North. 

• Great circle straight up. But bearing does not indicate 0 deg. 
• Great circle along section boundary. Bearing deviates a bit from O deg. But maybe is GIS inaccuracy?? Or this section edge is not aimed to true north. 
• Property Line via xy rectangle. All edges show 0 deg

What is correct North? Any of the candidates in this sample? 
 

-B

[Ben Beaumont]

@Benson ShawI think you're asking about grid convergence. In the State of Victoria in Australia, the meridian of the CRS is line of longitude 147deg East. This has a grid convergence of zero. In Metro Melbourne, at long 145 E, the grid convergence is -1.226deg. [I established that from an online geodetic calculator.] That means the True North point is slightly anticlockwise (East) from Grid North directly up the screen. So yes, technically, we could take that into consideration when displaying a North point on our drawings. As you've pointed out, the CRS is a projection of the earth at a given point and will inevitably be a distortion of the true geometry at a granular scale. The values are generally small enough to ignore. 

However I believe - if you're working in a CRS - Vectorworks already knows the grid convergence value at the location you've nominated. In other words, Vectorworks knows (quietly in the background) where True North is at that point. So a value you may place in that field will be in relation to True North. That would be useful if you wanted to rotate your plan to establish a convenient 'project north' - ie; pivot the World underneath your project a convenient amount to make your building square with the internal cartesian grid of Vectorworks. Your project can still be geolocated... but conveniently displayed orthogonally to your computer screen. 

For your Lat/Long above I get a grid convergence of -0.46 using the Geoscience Australia calculator. But you'll need to do your own due dilligence on that. 

I'm not a GIS professional - I'm self-taught (teaching) in GIS... so hopefully I'm on the money with my explanation here. 

[Benson Shaw]

Whoa! @Ben Beaumont  thanks for reply. So workflow questions. 
 

1. vwx prop line does not adjust to CRS north automatically?  It seems to accept drawing x,y. This is perhaps your Convergence???
 

2. To make x,y geometry conform to georef CRS, i have a choice:

         A. Rotate plan via the menu bar feature

         B. Rotate the georef world via the georef compass tool or the dialog. 
         C.  or something else?
 

I’m leaning to B. 

 

 Notes

• usa State Plane system divides each of the various states into one or several CRSs. Washington State has 2, a north and a south CRS.  Both use the Willamette Meridian as the “Principal” meridian.
•  I’m guessing that Conversion value is proportional to east/west distance from the Principal, and can be translated to a rotational value to match xy north for a smallish project site. (But what if you are working with a regional or international transit system!)
• This is a dense science!!!!!  I know so little but just want my drawings to conform.

-B

[Ben Beaumont]

@Benson ShawI'll need to come back to you on how the Property Line behaves. If one draws a Property Line at the meridian of a CRS (so it's true and square) and change the longitude of the Internal Origin, we might expect the Property to become distorted on transformation - but it doesn't behave that way. The insertion of the object is shifted, but the regular shape and rotation is preserved. The Property Line tool may not be Geo aware - it may not show convergence. It's a question for the engineers. 

 

The Rotate Plan feature is a bit like picking up your monitor and turning it like a steering wheel so you don't need to crank your head. It's of transient help only. 

I think a better way to 'hard wire' your project to a "project north" would be to use the key-in Angle to True North and/or the Geolocate compass tool (yes, your option B). That way you could 'set and forget' the dominant geometry of your project to the regular internal Vectorworks cartesian grid AND preserve the geolocation. 

[rudybeuc@gmail.com]

TEMP001.TIF

On 3/8/2023 at 5:18 PM, Ben Beaumont said:

@Benson ShawI'll need to come back to you on how the Property Line behaves. If one draws a Property Line at the meridian of a CRS (so it's true and square) and change the longitude of the Internal Origin, we might expect the Property to become distorted on transformation - but it doesn't behave that way. The insertion of the object is shifted, but the regular shape and rotation is preserved. The Property Line tool may not be Geo aware - it may not show convergence. It's a question for the engineers. 

 

The Rotate Plan feature is a bit like picking up your monitor and turning it like a steering wheel so you don't need to crank your head. It's of transient help only. 

I think a better way to 'hard wire' your project to a "project north" would be to use the key-in Angle to True North and/or the Geolocate compass tool (yes, your option B). That way you could 'set and forget' the dominant geometry of your project to the regular internal Vectorworks cartesian grid AND preserve the geolocation. 

 

So how would property line bearings work with that workflow?

 

Thanks,

Rudy Beuc

Edited March 31, 2023 by rudybeuc@gmail.com
corrected grammer...

[Benson Shaw]

Just amateur commentary based on what seems to be implied in thread above:

 

1. Geolocate the file to match survey or GIS conditions. Place internal and user origins near the site. Match all exist and new layers to this CRL and Projection. 
2. Acquire and place geostakes at coordinates of some significant point(s) in the property, esp one or more of the prop corners. This should be from a certified survey, out to 10 or 11 decimal points. GIS data from gEarth, county, city or USGS, etc should be close, but not good for legal or financial decisions.
3. Import, scale the tiff on a separate georeffed layer.  It’s hand drawn, and dims are only one or two decimals, and bearings are shown only to minutes, so is approximate geometry. Locate it so one corner matches a survey corner stake or GIS corner. 
4. Trace the prop boundary with rectangle or polyline, or polygon tools and convert to Prop Line. 
5. Edit to adjust lengths and bearings to match callouts on the tiff. Prop line edits are a bit tricky but you get the hang of it eventually.  If needed, move the entire propline object to make that corner point coincident again. 
6. Prop line object apparently is not georef sensitive. North of tiff and trace may not match georef north.  Eg, segments of the prop line showing correct length/bearings may not match rotation/location of georef objects.
7. If GIS prop lines are not aligned to the vwx prop line, use the geolocate>compass tool to rotate the georef world around the corner point to acquire best matchup. This correction should keep the georef geometry aligned with xyz of non georeffed geometry. 
    

That’s my take, anyway. 

-B