Calculate grid scale factor compatibly

[pkturner]

a new approach

This PR is a descendant of #1277 (Determine grid scale factor automatically) showing an approach that I like. It eliminates the matrix that compensates for shear in the projection. As a consequence, this version very compatible with previous releases of Mapper. It computes grid scale factor, while also permitting the user to make adjustments. It has a user setting to tailor the Georeferencing dialog to the user's knowledge.

the parallel

These enhancements to both user interface and code can be understood more easily, if certain parallels are kept in mind between parameters indicating orientation of the grid, and factors governing scale of the grid.

		orientation	scale
grid to model	computable from CRS	convergence	grid scale factor
model to terrain	user-specified	declination	auxiliary scale factor
grid to terrain	used in transformation between projected and map coordinates	grivation	combined scale factor
relationship		declination = grivation + convergence	combined scale factor = grid scale factor * auxiliary scale factor

As the table shows, convergence is analogous to grid scale factor in that both can be computed from the CRS and reference point. Similarly, declination is analogous to auxiliary scale factor as both are user-supplied.

Note, the “grid scale factor” as it appeared previously in Mapper, really meant “combined scale factor” and the more accurate term is used in both the new implementation and user interface.

The user-specified scale factor is called “auxiliary scale factor”, rather than “elevation scale factor” so as not to limit what users might do with it.

Earlier versions of Mapper provided for editing the "grid scale factor" which was effectively the combined scale factor, compensating for both the grid and any other needed adjustments. With this change, the grid compensation is automatic, and the user no longer sets the combined scale factor. Now the user sets the auxiliary scale factor. That relieves the user of determining and dealing with the grid scale factor.

the enhancement to Georeferencing

The enhancement to the Georeferencing class is modest.

• It computes the convergence and grid scale factor together.
• The methods get/setGridScaleFactor are renamed to get/setCombinedScaleFactor.
• get/setAuxiliaryScaleFactor methods are added.
• The auxiliary_scale_factor is saved/loaded as an attribute in .omap files.
• It adds an auxiliaryScaleFactorChanged signal.

compatibility

The new Georeferencing class maintains compatibility by using 'grid_scale_factor' in .omap files to save and restore the combined scale factor. It introduces an 'auxiliary_scale_factor' attribute into .omap files. This new attribute, if encountered by an earlier version of Mapper, is ignored with no ill effect.
When the new Georeferencing class loading from a file, the auxiliary scale factor is calculated if absent. I suppose it should always be re-calculated, as is done for declination (#1493).

use cases

Considering that there is no button to click that sets the scale factor to the calculated value, how does the user benefit from the changes?

new map
The primary case that benefits from this PR is a new map, because the grid scale factor is computed, and the auxiliary scale factor is simply 1.0. It is automatic, without the user explicitly choosing "automatic".

changed CRS
Another interesting case is when the CRS (or reference point) of an existing map is changed. The grid scale factor is re-computed, but the auxiliary scale factor is kept. This is obviously a good result if the auxiliary scale factor has been set to account for elevation. Thus, the combined scale factor is again automatic.

existing map
If the grid scale factor was set using an earlier Mapper, the value will appear unchanged in the Georeferencing dialog as “Combined scale factor”. (An Auxiliary scale factor will also be displayed, having the value which is consistent with the combined scale factor and the calculated grid scale factor.)

If a user wishes to take advantage of calculated grid scale factor for such a map, he/she may set the auxiliary scale factor to 1.0, using the Georeferencing dialog.

The effect of setting auxiliary scale factor is not trivial, as indicated when a dialog pops up, asking “Do you want to stretch/shrink the map content accordingly?” If “stretch/shrink” is declined, then the global coordinates of all map objects will be affected. This new dialog is analogous to the “Rotate map” dialog which appears if the user changes the declination.

taming the Georeferencing dialog

It can be understood from the description of this enhancement, that the benefit will occur gradually, as new maps are created. Note that the new feature does not depend on a feature flag being turned on in the Georeferencing object, or in the way it is saved to file.

At the same time, we are not content with having made the Georeferencing dialog bigger. We would like to give the user an option to avoid yet more subtlety around scale factors.

Noticing that the new Georeferencing does not require any user interaction to achieve its benefits, and gives all users less need to attend to scale factor, this PR proposes to remove the scale factor from the dialog, and give the user a setting to make the scale factors visible and editable.

[dg0yt]

👍

Regarding the georeferencing dialog, I'm fine with hiding the auxiliary scale factor field by default. However, wouldn't it be better to always show the auxiliary factor field when the current map has a value different from 1.0? After all, the user might be looking into the dialog to find an explanation for some observations, or for what he knew to be there as "Grid scale factor".
Or we could turn the grid compensation section into something collapsible/expandable, instead of using the settings dialog.

[pkturner]

@dg0yt

However, wouldn't it be better to always show the auxiliary factor field when the current map has a value different from 1.0?

One reason against that is that most existing maps will have an auxiliary scale factor different from 1.0, while at the same time the user will be doing well to not bother with it. Here's a typical case I expect.

factor	value
combined scale factor	1.0
grid scale factor	0.9999
auxiliary scale factor	1.0001

That's because the mapper has until now left the scale factor at 1.0. The new Georeferencing keeps this as the combined scale factor value, in order to retain both the scale of objects on paper and their lat/long on the globe. It calculates the grid scale factor, typically not 1.0, and consequently the auxiliary scale factor also cannot equal 1.0.

In such a case, showing the auxiliary scale factor would lead to unhelpful discontent.

Showing the auxiliary scale factor could have a minimum threshold before it would appear. I was lazy and didn't want to choose a threshold arbitrarily.

---

A weaker reason is that it's more comfortable for users if the form of the dialog doesn't vary when switching between maps.

[pkturner]

@dg0yt

Or we could turn the grid compensation section into something collapsible/expandable, instead of using the settings dialog.

That would be good, provided the collapsed/expanded state were kept as a user preference rather than as a property of the map.

I don't have a specific vision of how that should work in the Georeferencing dialog. If you would point me to what you have in mind, I'd be happy to implement it. Is there a case of another user preference in Mapper that is controlled apart from the settings dialog?

[dg0yt]

Or we could turn the grid compensation section into something collapsible/expandable, instead of using the settings dialog.

That would be good, provided the collapsed/expanded state were kept as a user preference rather than as a property of the map.

Yes, this is what I meant.

I don't have a specific vision of how that should work in the Georeferencing dialog. If you would point me to what you have in mind, I'd be happy to implement it. Is there a case of another user preference in Mapper that is controlled apart from the settings dialog?

We have some check boxes in the line symbol settings which also control the visibility of additional fields. The dependent widgets are kept in lists which are used when changing the state, e.g. for dashed lines:

mapper/src/gui/symbols/line_symbol_settings.cpp

Lines 686 to 715 in e1155d8

	const bool line_dashed = symbol->dashed && symbol->color;
	if (line_dashed)
	{
	for (auto undashed_widget : undashed_widget_list)
	{
	undashed_widget->setVisible(false);
	}
	for (auto dashed_widget : dashed_widget_list)
	{
	dashed_widget->setVisible(true);
	dashed_widget->setEnabled(line_active);
	}
	in_group_break_length_label->setEnabled(line_active && symbol->dashes_in_group > 1);
	in_group_break_length_edit->setEnabled(line_active && symbol->dashes_in_group > 1);
	half_outer_dashes_check->setEnabled(line_active && symbol->dashes_in_group == 1);
	}
	else
	{
	for (auto undashed_widget : undashed_widget_list)
	{
	undashed_widget->setVisible(true);
	undashed_widget->setEnabled(!symbol->mid_symbol->isEmpty());
	}
	show_at_least_one_symbol_check->setEnabled(show_at_least_one_symbol_check->isEnabled() && symbol->end_length > 0);
	for (auto dashed_widget : dashed_widget_list)
	{
	dashed_widget->setVisible(false);
	}
	}

For the start, use a "Grid compensation..." checkbox, a flat, checkable "Grid compensation..." push button or a label with a hyperlink. Later, we can override the normal checkbox marks or prefix the button/label text with a small triangle (▶ / ▼), or plus/minus ([+]/[-]). We must see what works best cross-platform.

[pkturner]

For the start, use a "Grid compensation..." checkbox, a flat, checkable "Grid compensation..." push button or a label with a hyperlink.

Thanks. I'll do that.

[dg0yt]

Looks good to me. Thank you very much!

[pkturner]

@dg0yt Thanks for all the guidance. It's refreshing to work with up-to-date coding standards.
Should I close this?

[dg0yt]

Thanks for all the guidance. It's refreshing to work with up-to-date coding standards.

Thanks for taking the time and for the explanations for your side.

Should I close this?

It will be closed automatically when I click merge. I just wanted to make sure there were no more changes pending.