[Qgis-us-user] Question on area calculations

[David Noble]

 I had similar trouble with the distance measure tool. I don't have details at hand, but my project in Spokane, WA area seemed fine. New project in Coeur d'Alene, ID area  was giving distances higher than Google Earth and Bing. I took the car and drove some of the routes, noting mileage posts, comparing to my odometer. I already knew my odometer was reading about 10% high. I found my mapping in Idaho needed about 65% correction! I suspect difference in projections and conversions when using different projection sources.
My advice is to verify in the field. Don't rely on aerial scaling.
I found I can adjust the map scale in the map composer. Under feature (individual?) property, the scale bar can be tweeked, using the inverse of the correction factor. I guess it makes the bar appear longer, thus when compared to map, lengths measure shorter. This probably has no effect on area measurements, unfortunately. I had feet numbers overlapping each other, so I manually covered them and inserted aprox. mileage numbers over the scale bar.
-Dave

    On Friday, April 13, 2018, 6:14:59 AM PDT, Randal Hale <rjhale at northrivergeographic.com> wrote:  
 
   
This has me puzzled. So I may have screwed up a very large job with incorrect acres and I was doing what I thought I needed to be doing. I'm probably doing this wrong - but if I'm doing this wrong there's a large chance more people are doing the same. 
 
 
This is all in QGIS 3.0.1
 
 
I have a forester who is working on a tract of land. There was a very hurried cruise they needed to do. 
 
 
Data is in WGS84 and set to EPSG:2240 in QGIS. 
 
 
Change the ellipsoid to None under project properties: 
 
    
   - using the identify button are 237 acres.    
 
   - I get roughly 194 acres with the measure tool   
 
 
Changing the ellipsoid to GRS1980 (which QGIS Defaults to): 
 
    
   - I get 196 acres with the identify tool.    
 
   -  I use the measure tool I get 194 acres
 
I am assuming the 194/196 measurements are the exact same and due to human error on my part. PostGIS gives me 196 acres. 
 
 
I guess (as I type this out) my correct course of action should be to leave the ellipsoid in GRS1980 and do my work - BUT - I thought I could drop back to none/planimetric if I set the projection to EPSG:2240 in QGIS.
 
Can anyone clue me in - I'm preparing for much yelling from the client in about 15 minutes as I was off about 40 acres which was a costly mistake. I'm not confident currently in my area measurements using QGIS and I need to get confident. 
 
 
Randy
 
 

 
 

 
 

 
 

 
 -- 
Randal Hale 
rjhale at northrivergeographic.com
https://www.northrivergeographic.com
(423)653-3611

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