QGIS » QGIS Application

I just checked and could not reproduce (yet).

OTF turned on:
With project CRS set to 4326 I get 2523339 in the identify dialog and the field calculator (opened from the attribute table)
With project CRS set to 20790 I get 2520002 in the identify dialog and the field calculator (opened from the attribute table)

Can you add a project file to the zip?

Problem finally solved in QGIS nightly build 55f8606!
Cheers

Sorry Giovanni, but the bug surfaced again in nightly build 30900e9!! It has exactly the same symptoms it had before. Strange...
Cheers

Hi Alvaro

You are right! Maybe the none/planimetric option should be selected as "default" for every new project.
Cheers

Hi Rhenriques, then this issue can be closed?
Best regards

Hi Alvaro

I suppose so. I've tested in several projects and values are always correctly calculated if 'none/planimetric' option is used, as expected.
Please ensure that this option is set as default in the next versions. No one wants miscalculated areas or any other wrong measurements in their tables!
Cheers and thank you.

There is a side-effect of setting the ellipsoid to "None/planimetric":
- Set the otf projection to a non-projected crs, eg wgs84 (epsg:4326)
- In the general tab set the canvas units to "meters"
- Try to measure a distance on the map - the distance is shown in degrees, not metres

There's some relevant discussion in issue #7441.

Nyall is right! I've only tested in projects with projected units. The settings are not even stable. Sometimes things change (units; ellipsoid) each time we open the Project Settings dialog (NB).
This issue must be changed to blocker due to the major implications it represents. You can have the best GIS software but if it's unable to calculate correctly, in a stable basis, a distance or an area, it becomes useless in real world projects. Strange because it all seem much more stable in 1.8. Another strange thing is that the 'preview value' of the 'field calculator' is mostly right. How is this possible?
Cheers

Hi, this patch fix the ellipsoid and units selection in the project properties dialog. The current settings are preserved when the dialog is showed and fix a bug to select the related ellipsoid of the current CRS (https://github.com/qgis/QGIS/pull/1071/files#diff-b335df9eb7a06c67dc9347624b7c3b53L1027).

I have not tested the measure tool, I guess it works fine for one so mature application ;-), but I tested the example of Rhenriques and It seems ok.

Best regards

Hi Giovanni, the pull request 'https://github.com/qgis/QGIS/pull/1071' has not yet been merged.
Do you think that it is not necessary to fix this issue ?

Best Regards
Alvaro

Alvaro Huarte wrote:

Hi Giovanni, the pull request 'https://github.com/qgis/QGIS/pull/1071' has not yet been merged.
Do you think that it is not necessary to fix this issue ?

Best Regards
Alvaro

it seems to me that master is returning right values as it is now, but I would really appreciate if you can give a look. Also it would e important to have the feedback of the original reporter.

Giovanni Manghi wrote:

Alvaro Huarte wrote:

Hi Giovanni, the pull request 'https://github.com/qgis/QGIS/pull/1071' has not yet been merged.
Do you think that it is not necessary to fix this issue ?

Best Regards
Alvaro

it seems to me that master is returning right values as it is now, but I would really appreciate if you can give a look. Also it would e important to have the feedback of the original reporter.

I think that 'https://github.com/ahuarte47/QGIS/commit/e0342dcf077c9be04b1de416abbc718b616d074d' fixes as least a bug searching the ellipsoid related to one CRS

Just tested in NB, code revision "d26831b" and is not right yet. If the original shape CRS is used in the project definition, the area is right. As soon as we change the project CRS to WGS84, the area calculation gets wrong. Same as before unfortunately.
Cheers

Correct!

IMHO, the correct solution would be that both the measurement tools and the field calculation functions should obey the same global set-up value regarding the ellipsoid choice. And the default ellipsoid should be the one used in the project default CRS. The use of planimetric measurement is a leftover from the days where you had to use a planimeter (http://en.wikipedia.org/wiki/Planimeter) to calculate areas on a paper map.

Would it be possible to the following choices ?:

"Use ellipsoid from default project CRS"
"Use specific ellipsoid" (together with a "ellipsoid chooser" dialogue)
"Use planimetric calculations"

Although the main point is to have the different tools to report the same area/length measurements

Regards
Bo Victor Thomsen
Aestas-GIS
Denmark

I am getting quite confused by the discussion as there have been various issues mentioned.

Can you please clearly describe (with the help of attached QUAD project) in what case things get wrong and what is the expected outcome?
- on-the-fly projections on/off
- project CRS
- project measurement ellipsoid
- tool used (measure tool / identify tool / field calculator / all)
- anything else that affects calculation?

The only issue I can see right now is with OTF projections on, project CRS: wgs84, ellipsoid: none.
The identify tool says the area is 31 227 946 258,332 km² which is completely off.

As a bonus, with these settings above, after opening the project properties dialog, the measurement ellipsoid says WGS 84 which is wrong (planimetric is being used).

Is this the problem or are there more cases to fix?

Is this the problem or are there more cases to fix?

Hi Martin, I resume here what we discussed in private. Anyway it seems to me that if the issue of the completely wrong computed values is solved then the actual implementation make sense, and this introduce a different behavior since 1.8. In the future the approach used in the "add/export geometry columns" tool in the vector menu may be used.

Giovanni:

take as input a 10x10km square in a projected CRS

Hi Martin, I resume here what we discussed privately.

qgis 1.8:

the ellipsoid for measurements option was defined in the qgis general
options, wgs84 by default

now add your square, otfr off, compute area with field calculator -> 100sqkm

otfr on, project CRS same as layer CRS, compute area with field
calculator -> 100sqkm

otfr on, project CRS different from the layer CRS (geographic or
projected does not matter), compute area with field calculator ->
100sqkm

qgis master:

the ellipsoid for measurements option is defined in the project
options, wgs84 by default

now add your square, otfr off, compute area with field calculator -> 100sqkm

otfr on, project CRS same as layer CRS, compute area with field
calculator -> 100sqkm

otfr on, project CRS different from the layer CRS (geographic or
projected does not matter), compute area with field calculator ->
NOT 100sqkm... but a close value (as expected?). Problem is that
sometimes the result is a completely wrong value (not even close).

If the user changes the ellipsoid for measurements to "planar/none"
that the results are equals to the ones in qgis 1.8

Martin:

This is expected - or at least for me... instead of measuring the area
in layer's coordinate system, it will reproject it to lat/lon coords
on ellipsoid and calculate the area on ellipsoid - a relatively small
difference is understandable.

But maybe not what the users want?

Giovanni:

I agree, but this change was undocumented and probably unexpected by
many users, so
is leading to confusions.

the feedback I have is that most users expect to have the area/length
of layers in a projected CRS to be always
computed in the layers CRS.

But I also agree that the actual behaviour of qgis make sense (for the
more experienced user), so the above could
eventually be added as an option in general or project properties.

maybe the approach that can be used (as option in the project/general
options) is the one already available in the
"add/export geometry columns" tool in the vector menu.

Martin:

This looks like a good option for the future - should be relatively clear.

It seems that planimetric computation in layer's CRS is the usual
assumption. Then there are people with unprojected data (e.g. from
GPS) trying to measure something, for them it makes more sense to have
area computed on ellipsoid rather than area in squared degrees :-)
.... all the other cases are probably "advanced" and the user should
have a choice to decide how to do the measuring.

Current state of measuring:
- ellipsoid "none" - consistently returning 2.520 km^2 (with project's CRS being EPSG:20790, WGS84 or with OTFR off)
- ellipsoid "wgs84" - consistently returning 2.523 km^2 (with project's CRS being EPSG:20790 or WGS84)

Consistently means the same result for identify tool, $area and measure tool.

The only outlier is measure tool for project's CRS being WGS84 and ellipsoid "none" - the reported area is ~3.4 km2 due to measuring in degrees and then converting that value with a fixed degree-to-meters ratio, so that is acceptable (IMHO)

In case of any further issues please open a new bug as all the comments in this issue make it hard to follow.

Martin Dobias wrote:

In case of any further issues please open a new bug as all the comments in this issue make it hard to follow.

Hi Martin, does your fix apply also to the field calculator?

cheers!

No, it fixes only wrong value in identify tool. I am not aware of any issue in field calculator.

Hi Martin, there is an issue commented in #9060-14 that I think fixed in https://github.com/qgis/QGIS/pull/1071

Cheers!!!

Martin Dobias wrote:

No, it fixes only wrong value in identify tool. I am not aware of any issue in field calculator.

I'm afraid also the FC is affected.

I'm paying attention to every single NB about this issue. Changes are not committed yet in the NB: f06457b. Changing flags in the project settings, about units and projection, are not honoured yet too. This happens under circumstances that are not easy to replicate sometimes, but mainly are related with turning on or off the OTFR. If we change to meters and planimetric, for instance, things change again the next time we open the project dialogue settings to default.
While using the 2.01 version, in projects with a high degree of responsibility, I use a shape with a perfect 1 sq km that I copy to every polygon shape, to ensure that I got the areas right in the field calculator. Very annoying...
Cheers

Hi Rhenriques, this pull (https://github.com/qgis/QGIS/pull/1071) (not merged) attempts to correct that behavior that are not respected previous settings.

Can you test it ?

I do not feel confident merging the proposed PR this close before the release as there are several non-trivial changes where I cannot foresee their impact. It needs more testing to see if various possible cases are covered.

Martin Dobias wrote:

I do not feel confident merging the proposed PR this close before the release as there are several non-trivial changes where I cannot foresee their impact. It needs more testing to see if various possible cases are covered.

ok, then there are two minor changes:

https://github.com/ahuarte47/QGIS/commit/4b352c342704f46e43ecee39e9c0e5bb4662e8d0
https://github.com/qgis/QGIS/pull/1054

Merged the two commits from Alvaro (35791aa and c784c0)

The real issue with the project properties and setting of on-the-fly reprojection flag, CRS, map units, ellipsoid is that it is poorly designed and the user has no clue which settings are forced by QGIS and which of them are recommended with the possibility to override them.

Martin Dobias wrote:

I do not feel confident merging the proposed PR this close before the release as there are several non-trivial changes where I cannot foresee their impact. It needs more testing to see if various possible cases are covered.

Hi Martin,

I just tested master with a code revision prior of your last commit.

This is what I see using the field calculator.

The input is a 1x1km grid in a projected CRS (tested 3763)

1) otfr off area = 1000000
2) otfr on, project CRS same as layer area = 1002464.5100 - 1002535.8531
3) otfr on, project CRS different from layer CRS (projected or geographic) area = 1002464.5100 - 1002535.8531

the Field Calculator preview always show 1000000

Maybe 3) makes sense, but 2) does not seems to.

The behaviour that Giovanni described is exactly what I am obtaining too. As I told before, and as Giovanni wrote, the strange thing is that field calculator always show the correct value in the preview.

Cheers

Giovanni Manghi wrote:

The input is a 1x1km grid in a projected CRS (tested 3763)

1) otfr off area = 1000000
2) otfr on, project CRS same as layer area = 1002464.5100 - 1002535.8531
3) otfr on, project CRS different from layer CRS (projected or geographic) area = 1002464.5100 - 1002535.8531

the Field Calculator preview always show 1000000

Maybe 3) makes sense, but 2) does not seems to.

For 2) and 3), if you did not use "none / planimetric" ellipsoid for measuring, the result makes sense because the measurements are done on ellipsoid. Only with "none / planimetric" ellipsoid it will give you 1000000. I am not saying this logic is optimal, it is simply the behavior that someone has defined previously.

Btw. interesting that you can see preview in field calculator - for me the preview for "$area" is always empty.

For 2) and 3), if you did not use "none / planimetric" ellipsoid for measuring, the result makes sense because the measurements are done on ellipsoid. Only with "none / planimetric" ellipsoid it will give you 1000000. I am not saying this logic is optimal, it is simply the behavior that someone has defined previously.

as discussed previously this is of course right, and makes sense for power users, but if wgs84 is the defaut ellispoid (as it is) this can be confusing for most of the users, especially because it is change since 1.8.

I'm really not sure what would be the best option at this point.

Btw. interesting that you can see preview in field calculator - for me the preview for "$area" is always empty.

:)

Giovanni Manghi wrote:

For 2) and 3), if you did not use "none / planimetric" ellipsoid for measuring, the result makes sense because the measurements are done on ellipsoid. Only with "none / planimetric" ellipsoid it will give you 1000000. I am not saying this logic is optimal, it is simply the behavior that someone has defined previously.

as discussed previously this is of course right, and makes sense for power users, but if wgs84 is the defaut ellispoid (as it is) this can be confusing for most of the users, especially because it is change since 1.8.

I'm really not sure what would be the best option at this point.

Me neither - but it does not make sense to do any radical changes right now.

It is clear that for future versions we need to rethink how we will deal will measuring settings, map units and related things to clean the mess we have right now - a solution with sensible defaults and logic where one gets expected results all the time.

I confirm, the $area preview shows the result

Another fix (d92a0e) - make sure to show preview of "$area" + make the preview consistent with field calculator results

Here are my thoughts on this

The measure tool dialog for me for selecting Ellipsoid is superfluous in the General Tab, units should be selected directly on the Measure tool that way I can go from m to km to NM to ft if I wanted.

- If project uses a Projected coordinate system then the measure tool should give me the measurements using that PCS by default in the units I have previously selected, only meters, ft and NM should be shown as options (BTW miles should be added as its used in the USA) no degrees options here as it doesn't make sense

- If the project is set to using a Geographic coordinate system then measurements should be used by selecting the ellipsoid from the definition, if OTF is off then only degrees should be shown as option if OTF is on only m,ft, NM should be shown. It makes absolutely no sense to have the QGIS project use Airy 1830 for measure calculations if WGS 84 is used for the project

In this scenario if I have a layer in WGS UTM

a) If project is set to use UTM then I will can measure in m, ft, etc
b) If project is in WGS then I can get geodetic distance in m, ft, etc

Field calculator should work similar

Antonio Locandro wrote:

Here are my thoughts on this

The measure tool dialog for me for selecting Ellipsoid is superfluous in the General Tab, units should be selected directly on the Measure tool that way I can go from m to km to NM to ft if I wanted.

- If project uses a Projected coordinate system then the measure tool should give me the measurements using that PCS by default in the units I have previously selected, only meters, ft and NM should be shown as options (BTW miles should be added as its used in the USA) no degrees options here as it doesn't make sense

- If the project is set to using a Geographic coordinate system then measurements should be used by selecting the ellipsoid from the definition, if OTF is off then only degrees should be shown as option if OTF is on only m,ft, NM should be shown. It makes absolutely no sense to have the QGIS project use Airy 1830 for measure calculations if WGS 84 is used for the project

In this scenario if I have a layer in WGS UTM

a) If project is set to use UTM then I will can measure in m, ft, etc
b) If project is in WGS then I can get geodetic distance in m, ft, etc

Field calculator should work similar

Hi Antonio, please send your notes also the dev mailing list, we need to figure out a way to solve this issues.