Deriving areas and perimeters for drawings/polygons in their table.

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#02-Jul-24 15:14

Hello - I am new to MANIFOLD, and not a GIS pro. Can anyone help me with creating area (ha) and perimeter (m) fields for drawings/polygons. My projection/datum being used is WGS84/Pseudo-Mercator (EPSG:3857) with a base raster from UK Ordinance Survey. I seem unable to find anything in the MANIFOLD manual or videos to help me - though I have not spent days on this.

635 post(s)

#02-Jul-24 17:09

Welcome.

Instead of trying to describe it in words, I recorded my second attempt of doing this myself. I also attached the project. I think I did it right.

https://youtu.be/viMZF7bNrIM

Attachments:
m9_add_area_ha_perimeter_m.mxb

oeaulong

531 post(s)

#03-Jul-24 00:00

Nice tutorial. What software are you using for the key capture overlay?

635 post(s)

#03-Jul-24 13:21

Keyviz. I found and installed it just minutes before recording. It worked OK for the first time. I could not change the position of the “viz” as I would have wanted to and there is a delay between the keypress and the “viz”. Maybe the latter is configurable idk.

oeaulong

531 post(s)

#03-Jul-24 15:12

Thank you.

Speedy122 post(s)

#15-Jul-24 10:33

Thank you - this has been a great help. Seems Manifold is much more complex in this regard than I recall Arc and Mapinfo being...

hphillips33 post(s)

#03-Jul-24 15:27

I recognize that your question regarded how to create and populate area and perimeter field for the data table for a drawing and rk gave an answer for that question. My comment pertains to the contents of those fields. I don't believe Web Mercator is a good projection for accurate measurement of length and area much away from the equator. At more northerly latitudes, say the US or UK, most mapping agencies would probably use a coordinate system based on a transverse mercator or conformal conic projection for better measurement accuracy.

635 post(s)

#03-Jul-24 23:52

Yes. And it is the job of CoordMeasureMake(<system>, <unit>, <geodetic>) to make the calculation geodetic when the last argument is TRUE. I set it to TRUE in the video and at the end, I show the result with FALSE too.

And you are absolutely correct. I have been in the situation where the input to a system with simple custom area calculation changed from conformal conic to web mercator and confusion wasnt cleared too quickly.