dhis2-devs team mailing list archive

[Knut Staring <knutst@xxxxxxxxx>]

Moving this discussion to the mailing list.

2010/8/23 Lars Helge Øverland <larshelge@xxxxxxxxx>:
>
> On Mon, Aug 23, 2010 at 5:11 PM, Knut Staring <knutst@xxxxxxxxx> wrote:
>>
>> Hi Lars, could you please update
>>
>> dhis-2/dhis-services/dhis-service-importexport/src/main/resources/transform?
>
> I thought the conclusion of Jason's decimal saga was that we were not going
> to cut the decimals?

Nope. Let me set out the issues in detail:

1) One concern is that DHIS2 should be able to work as a repository of
OrgUnit info, including shapes, and potentially also offer this data
through web services to clients other than the OpenHealthMapper (OHM).
Therefore, we should not reduce the precision of the orginal GIS data.
For this, we may need to store the original files separately from the
OrgUnit table. However, I am not fully convinced we need this for now
- it could be a blueprint for 2.0.6.

2) The most important concern is the need to supply the OHM client
with GeoJSON files that are not too large. GeoJSON files are 25-70%
larger than shapefiles. We cannot ask  people with slow connections to
download 15 MB just to see a map - and many browsers will choke under
the burden of processing such large files.

3) Furthermore, the shape-to-GML conversion algorithm used by ogr2ogr
(which is a very good tool), ALWAYS results in each coordinate having
15 decimals, regardless of the precision of the original file, whereas
the shape-to-GeoJSON converter does not do that. I illustrated that
clearly several times in this discussion. Our method of importing from
shapefiles triples the size of what we store, transmit and process.
Let me illustrate:

I have a shapefile with all the countries in the world. This is 6 MB.
Converting to GeoJSON makes it 9.95 MB. Converting to GML makes it
14.7 MB.

Here are the values for the first coordinate using ogr2ogr in three
different ways:
Shape-to-GeoJSON  -69.882233
GeoJSON-to GML    -69.882232999999999
Shape-to-GML         -69.882232666015625

Rounding to 4 decimals would preserve precision down to 10 m,  which
is sufficient for our purposes (we are not using the files as input
for building construction), but 5 or 6 should also work.

Knut

>>
>> Knut
>>
>> On Tue, Aug 17, 2010 at 11:59 AM, Knut Staring <knutst@xxxxxxxxx> wrote:
>> > Hi guys,
>> >
>> > In line with our earlier discussions, I have updated the gml2dxf
>> > transformation to round to 4 decimals (corresponding to an accuracy of
>> > 10 meters), which will reduce the geojson for polygons significantly.
>> > We may want to keep one more decimal in the case of points, should not
>> > be too hard.
>> >
>> > However, I am very rusty in terms of committing now with branches and
>> > merging and stuff - could one of you pls help out?
>> >
>> > The file is in
>> > dhis-2\dhis-services\dhis-service-importexport\target\transform
>> >
>> > Thanks,
>> > Knut
>> >
>>
>>
>>
>> --
>> Cheers,
>> Knut Staring
>
>



-- 
Cheers,
Knut Staring