[mapserver-dev] RFC 89 Layer Geomtransform: Call for vote

[Daniel Morissette]

Hi Steve,

Is it possible that you are missing SIZEUNITS and UNITS?

The WIDTH of your lines is specified in SIZEUNITS, and OTOH the UNITS 
param I was alluding to is used to indicate the units of the source 
data. The doc says that it is currently used only for inline features 
(is this true?), but with this RFC it would become more widely used.


On 13-02-20 11:09 PM, Stephen Woodbridge wrote:
> On 2/20/2013 10:11 PM, Daniel Morissette wrote:
>> On 13-02-20 1:36 PM, Alan Boudreault wrote:
>>> Initially, that's what I wanted to avoid: adding new GEOMTRANSFORMUNITS.
>>> We already have UNITS, TOLERANCEUNITS and SIZEUNITS. I would tend to
>>> force the user to use UNITS with GEOMTRANSFORM. I'm not necessary right
>>> though.
>>>
>>
>> Thinking about this some more and out loud:
>>
>> - unless I'm mistaken, the UNITS value will matter in this context only
>> if you use the [map_cellsize] variable in your GEOMTRANSFORM expression.
>> In this case the UNITS setting is used to convert the map->cellsize from
>> the map output units to the units of the source data
>>
>> - if you don't use the [map_cellsize] variable then the args (e.g. the
>> simplify or buffer tolerance)) are passed directly to the corresponding
>> function (GEOS or other), so the arg value is assumed to be in the same
>> units as the source data (whatever it is) and the UNITS setting has no
>> impact in this case
>>
>> - there is currently no way in MapServer to set UNITS to a null/unset
>> value, so we have no way to know if UNITS was explicitly set in the
>> mapfile or not (the default units setting is meters)
>>
>> - adding a GEOMTRANSFORMUNITS param (presumably to add the ability to
>> pass the tolerance args in arbitrary units instead of in the data
>> source's units) will just complicate the issue and not solve the core
>> issue which is to determine in which units the original data source
>> coordinates are in order to convert the map>cellsize from map output
>> units to layer data source units for use in the [map_cellsize] variable
>>
>>
>> If the above are true, then a simple solution would be to always use the
>> layer->units value and never try to lookup of the projection units. We
>> should document the requirement to set UNITS together with the
>> [map_cellsize] variable docs (at the same place). If one chooses to use
>> [map_cellsize] in their GEOMTRANSFORM expression then they MUST also
>> explicitly set UNITS in the layer to the correct value (corresponding to
>> the UNITS of the raw vector data source).
>
> Daniel,
>
> This explanation is very clear and helps a lot - thank you.
>
> I agree with all of your points except for the part that if you set
> [map_cellsize] that you must set UNITS to reflect the raw source data
> units. This breaks the following use case.
>
> I have street data in degrees,
> I want to street widths to be rendered in meters
> (the proceeding is very common) and
> I need to apply a GEOTRANSFORM using [map_cellsize] or not.
>
> Your requirement means that I can not use different UNITS for the WIDTH,
> the GEOTRANSFORM with [map_cellsize] and my source data!
>
> OK, so maybe this is not a case anyone would want to use, I just don't
> know. Maybe we should provide a way to specify the source data units if
> that is what is unknown and needed?
>
> I'm not trying to make this more complicated than it needs to be and I
> think that there is significant value even if it isn't supported for all
> possible cases.
>
> Thanks,
>    -Steve W
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-- 
Daniel Morissette
http://www.mapgears.com/
Provider of Professional MapServer Support since 2000