[Qgis-us-user] Question on area calculations

[Craig Campbell]

My understanding is that you need to be in a projection where the units for that projection are in meters or feet before you will get accurate distance and area measurements.  Try using a UTM Zone that fits your area and the native values are in meters.

From: Qgis-us-user [mailto:qgis-us-user-bounces at lists.osgeo.org] On Behalf Of David Noble
Sent: Saturday, April 14, 2018 1:06 PM
To: QGIS-us-user at lists.osgeo.org; Randal Hale
Subject: Re: [Qgis-us-user] Question on area calculations

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<mailto: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<mailto:rjhale at northrivergeographic.com>

https://www.northrivergeographic.com

(423)653-3611


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