Clarification of false_easting / false_northing

[heiko.klein]

The explanation of false_easting/northing has left us often puzzled weather false_easting should be added to the existing x-axis values, or subtracted. To clarify this, I suggest the following modification to Table F.1:

false_easting

The value applied to all abscissa values in the rectangular coordinates for a map projection. This value frequently is assigned to eliminate negative numbers. Expressed in the unit of the coordinate variable identified by the standard name projection_x_coordinate. The formula to convert from a projection-axis value without false_easting (x0) to a projection-axis value with false_easting is (xf) is:
x0 = xf - false_easting

false_northing

The value applied to all ordinate values in the rectangular coordinates for a map projection. This value frequently is assigned to eliminate negative numbers. Expressed in the unit of the coordinate variable identified by the standard name projection_y_coordinate. The formula to convert from a projection-axis value without false_northing (y0) to a projection-axis value with false_northing is (yf) is then:
y0 = yf - false_northing

[caron]

Hi Heiko:

I like this clarification. However, the definitions seem somewhat circular. From my POV a "projection-axis value without false_easting" is the coordinate of the projection, while a "projection-axis value with false_easting" is the coordinate value in the file. The formula is correct.

Also, perhaps we should recommend to not use false_easting/northing when possible, ie just put the correct values into the file. It seems like an archaic practice from before we discovered negetive numbers.

John

[jonathan]

Dear Heiko

Thanks for opening this ticket. I have changed its status to a defect ticket, because I believe your intention is to clarify the text of the document, not the change the convention. A defect ticket is accepted by default, if there are no unanswered objections. If that's not the idea, please feel free to change its status back to enhancement!

I think it would be better to say "coordinate value" instead of "projection-axis value". Regarding John's comment, perhaps the distinction is between the coordinate value from the transformation without applying the offset, and the coordinate value stored in the coordinate variable after applying the offset.

Cheers

Jonathan

[heiko.klein]

Dear John and Jonathan,

thanks for your feedback. It wasn't clear to me which type of ticket to use, but with your explanation, I agree to a defect ticket. I agree, too, that false_easting/northing should be marked as deprecated - but I don't know how deprecations work in CF. I try to re-formulate the sentence to:

The value applied to all abscissa values in the rectangular coordinates for a map projection. This value was historically assigned to eliminate negative numbers. Expressed in the unit of the coordinate variable identified by the standard name projection_x_coordinate. The formula to convert from the coordinate value as written in the projection_x_axis (xf) to a value (x0) used in a transformation without false_easting, i.e. false_easting=0 is:
x0 = xf - false_easting

(and respectively for the false_northing sentence)

Heiko

[jonathan]

Dear Heiko

Thanks. I think that deprecation would be a change to the convention, and I expect that it is commonly used e.g. in a national grid for mapping. So I would not imply this feature is out of date. I would suggest "Applied to all abscissa values in the rectangular coordinates for a map projection in order to eliminate negative numbers. Expressed ...".

Best wishes

Jonathan

[heiko.klein]

Dear Jonathan,

from this discussion, and from an internal one, it seems like software-developers don't like the false_easting, while users do. Therefore, I agree to your re-formulation:

Applied to all abscissa values in the rectangular coordinates for a map projection in order to eliminate negative numbers. Expressed in the unit of the coordinate variable identified by the standard name projection_x_coordinate. The formula to convert from the coordinate value as written in the projection_x_axis (xf) to a value (x0) used in a transformation without false_easting, i.e. false_easting=0 is:
x0 = xf - false_easting

(and respectively for the false_northing sentence)

Heiko

[davidhassell]

Dear Heiko,

I, too, support this clarification, but I'm confused by the term "projection_x_axis". Do you mean "projection_x_coordinate" instead? That would make sense to me.

All the best,

David

[heiko.klein]

Dear David,

thanks for the correction, you are right, I meant projection_x_coordinate instead of projection_x_axis. The updated version is then:

Applied to all abscissa values in the rectangular coordinates for a map projection in order to eliminate negative numbers. Expressed in the unit of the coordinate variable identified by the standard name projection_x_coordinate. The formula to convert from the coordinate value as written in the projection_x_coordinate (xf) to a value (x0) used in a transformation without false_easting, i.e. false_easting=0 is:
x0 = xf - false_easting

(and respectively for the false_northing sentence)

Heiko

[jonathan]

Dear David and Heiko

Thanks for the correction. Since it's a defect ticket and no-one objected to it earlier, I think we can regard it as accepted now.

Cheers

Jonathan