[gdal-dev] PGeo: reading circular arcs - how?

[Stefan Keller]

Hi Ragi

Thank you very much!
Let's see if there's a hint.

Yours, S.


2010/6/10 Ragi Burhum <ragi at burhum.com>:
> Hello Stefan,
> Sorry for the delay in the reply (I am trying to clean up my e-mail today).
> Answers inline below
>
> On Sun, Jun 6, 2010 at 7:02 AM, Stefan Keller <sfkeller at gmail.com> wrote:
>>
>> Hi Ragi
>>
>> Thank you for the tips.
>>
>> I browsed through the pages you referred to but I somehow did not find
>> neither a formula nor the meaning of the parameters.
>>
>
> http://resources.esri.com/help/9.3/ArcGISDesktop/ArcObjects/esriGeometry/IPolycurve_Densify.htm
> Basically, there is a call to Densify. As far as the formula/algorithm to
> create those lines, well, look for Douglas Peucker.
>
>> Keep in mind that I am reading the binary format of PGeo and I am
>> focussing on circular arcs only. The beginning of the encoding binary
>> block elements are already clear.
>>
>
> Great! less to worry about :)
>
>>
>> What's confusing is that I would expect as attributes just a starting
>> point, a midpoint and an endpoint. But what's somehow there is s
>> startpoint and followed by 4 or more attributes.
>>
>
> If I remember correctly, serialization of curves changed between 9.2 to 9.3.
> So you need to know what GeoDatabase version you are reading (it is on one
> of the metadata tables).
> As far as what those attributes are, well, honestly, there was a reason to
> my "not fun" comment.
> You see, I don't think you will find a description of those attributes, but
> you can make an educated guess.
> If you look at the different ways to construct a Circular in ArcObjects
> http://resources.esri.com/help/9.3/ArcGISDesktop/ArcObjects/esriGeometry/IConstructCircularArc2.htm
> you will understand why it can be a pain. I don't know which attributes they
> are serializing, but it definitely is the parameters of one of the listed
> methods.
> Good luck,
> - Ragi
>
>>
>> Yours,
>> Stefan
>>
>>
>> 2010/6/5 Ragi Burhum <ragi at burhum.com>:
>> >
>> >> From: Stefan Keller <sfkeller at gmail.com>
>> >> Subject: [gdal-dev] PGeo: reading circular arcs - how?
>> >> To: gdal-dev at lists.osgeo.org
>> >>
>> >>
>> >> Hi all
>> >>
>> >> I have an ESRI Personal GeoDatabase .mdb file (PGeo) which I would
>> >> like to read and convert e.g. to Shapefile.
>> >> The PGeo file contains "circular arc"s (or circular curve) and since
>> >> OGR only supports linestrings they should be segmented (similar to
>> >> http://www.gdal.org/ogr/drv_ili.html ).
>> >>
>> >> Now it turned out that the output is wrong: It seems that at least the
>> >> midpoint(s) are misinterepreted.
>> >>
>> >> In fact, when looking at the source (line 430 ff.) there's a TODO
>> >> there:
>> >>
>> >> http://trac.osgeo.org/gdal/browser/trunk/gdal/ogr/ogrsf_frmts/pgeo/ogrpgeolayer.cpp
>> >>
>> >> ESRI user doc says at
>> >>
>> >> http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//002t0000000m000000.htm
>> >> "Circular arc curves use a mathematical formula to draw the curve. If
>> >> you export a geodatabase feature class containing circular
>> >> arc curve features to a shapefile, the curved features are transformed
>> >> to simple line features with closely spaced vertices to capture the
>> >> curved shape".
>> >>
>> >> => Does anybody know this formula
>> >
>> >
>> > http://resources.esri.com/help/9.3/ArcGISEngine/java/api/arcobjects/com/esri/arcgis/geometry/IPolycurve.html#densify(double,%20double)
>> >
>> >
>> >>  and/or the meaning of the
>> >> additional attributes in the byte stream?
>> >>
>> >
>> > depends on the type of curve. It could be a bezier curve, elliptical arc
>> > or circular arc. if you look at the coclasses of Icurve
>> > http://edndoc.esri.com/arcobjects/9.2/ComponentHelp/esrigeometry/ICurve.htm
>> > you will find attributes defined on those specific types that are most
>> > likely the serialized bytes.
>> >
>> > not fun.
>> >
>> > good luck,
>> >
>> > Ragi_______________________________________________
>> > gdal-dev mailing list
>> > gdal-dev at lists.osgeo.org
>> > http://lists.osgeo.org/mailman/listinfo/gdal-dev
>> >
>
>