# Perpendicular line construction

Global Mapper UserTrusted User Posts: 452
edited June 2014
Mike,

When creating a line, then using right-click menu:
Create Perpendicular Lines ...

I would expect lines to be drawn across the line at a bearing of 90 degrees ahead of the line.

I am seeing lines drawn at a bearing of about 67.225 degrees behind the line bearing.
The "perpendicular" line bearing varies with base line bearing, and with distance along line.
Tabulated below are the results of my experimentation:
``````Line    Line-90deg    Brg@01km    Brg@10km    Brg@20km
0    270    247.1863    247.1730    247.1581
30    300    277.2233    277.2248    277.2266
45    315    292.2500    292.2590    292.2693
60    330    307.2643    307.2893    307.2982
90      0    337.2826    337.3087    337.3378
135     45     22.3118     22.3397     22.3705
180     90     67.2514     67.2648     67.2796
225    135    112.1863    112.1773    112.1675
270    180    157.1507    157.1246    157.0956
315    225    202.1222    202.0943    202.0631
``````
I'll see if I can attach the workspace and a screen capture.

Are we seeing the effects of a projection? I'm using UTMz12 in Arizona (32.5N),
so I would expect a perpendicular to be relative to the UTM grid,
rather than relative to a geographic or non-rectangular projection.

I first attempted this operation on a rectangular grid near the south end of South America (51S),
using an Argentine rectangular projection that lines up nicely with client data,
and the results were even more skewed compared to my expectations.

Please align my expectations to what is really happening ;-)

I have accomplished my task in Excel, so there is no urgency to this report.
I simply calculate the slope of the reference line (dX/dY)
and apply it to the perpendicular lines with a slope of (dY/dX).

Mykle

edited June 2014
Mykle,

I took a look and found that the bearing calculation method (true north, magnetic north, or grid north) for a segment and the method used to get the points projected at the perpendicular to that were not necessarily the same. The perpendicular lines should now depend on the bearing mode selected on the Measure Tool right-click menu.

I have placed new builds with the latest changes at the links below:

Simply download the .zip file for the build that you are using and extract over your existing installation files. The likely locations to extract to are as follows:

32-bit: C:\Program Files (x86)\GlobalMapper15
64-bit: C:\Program Files\GlobalMapper15_64bit

Thanks,

Mike
Global Mapper Guru
geohelp@bluemarblegeo.com
Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
• Global Mapper User Trusted User Posts: 452
edited June 2014
That's perfect, thanks.

Would the Sample Spacing Setup dialog be an appropriate place to mention the currently selected bearing mode?
Methinks it would help (although I don't yet appreciate how it affects calculation of perpendicular lines).
If it isn't going to affect how the user sees the results, then it would not be needed.

Is it worth adding a "choice of angle" to the To Do list? I can think of times when something other than a 90 degree cross-line are appropriate.

Thanks!
Mykle
edited June 2014
Mykle,

The bearing might seem out of place on the Sample Spacing Setup dialog given that the dialog is used in several different places for other things that have nothing at all to do with the bearing mode. In general the bearing mode itself won't be too significant if you aren't using a distorted projection, it was just important that the same mode was used internally for creating the perpendicular lines.

Yes I can add an option to select the angle to the todo list. Right now you could create all of the perpendicular lines, then immediately select to rotate them after they are created and rotated to the appropriate angle.

Thanks,

Mike
Global Mapper Guru
geohelp@bluemarblegeo.com
Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation