dhis2-devs team mailing list archive

["Lemarchand, Johan" <lemarchandjo@xxxxxxx>]

Hi all,

The number of decimals is not really the issue. If you use 6 decimals, it is already enough for the type of GIS application we are interested in. The use of 15 decimals will not change a lot the precision of your map and it is not really necessary. 
0 decimal places = approx. 112 km (70 miles) (Precision depending on the latitude)
3 decimal places = approx 111 m (365 feet)
6 decimal places = < 0.3 m (< 1 foot)

The maps used by the system are not that accurate anyway to be more precise than 6 decimal places because there are not very large scale maps (1:1 000 or 1:500). There are medium scale maps 1:50 000 or 1:100 000 or small scale maps.

The issue is more the cartographic generalization and the fact that it is not preserving all intricate geographical or other cartographic details. It is necessary to run the generalization process in order to use the GeoJSON format, but it removes a lot of data and simplifies it as well. As a significant amount of data is lost in the process, the output files are not relevant regarding purpose and scale and the simplified GeoJSON files can't really be used in a GIS.

Johan

 

-----Original Message-----
From: dhis2-devs-bounces+lemarchandjo=who.int@xxxxxxxxxxxxxxxxxxx [mailto:dhis2-devs-bounces+lemarchandjo=who.int@xxxxxxxxxxxxxxxxxxx] On Behalf Of Knut Staring
Sent: 26 July 2010 10:17
To: Bob Jolliffe
Cc: dhis2-devs@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Dhis2-devs] Info on GIS development

On Mon, Jul 26, 2010 at 9:38 AM, Bob Jolliffe <bobjolliffe@xxxxxxxxx> wrote:
> Hi Jason
>
> On 26 July 2010 04:49, Jason Pickering <jason.p.pickering@xxxxxxxxx> wrote:
>> Hi Knut,
>>
>>> It may be that we want to use DHIS as both a repository with full
>>> precision (though not ridiculously artifical ones like 15 decimal
>>> lat/lon) and have a faster way of renderin. But for a repo, I think
>>> something like PostGIS is in order. Or we could just store things as
>>> GML...
>>
>> Well, this is really the issue. If DHIS is going to be a repository,
>> any self-respecting GIS geek would not use it if the application
>> clipped precision. Although a few meters is not significant in terms
>> of rendering a map, it may cause havoc on certain datasets,
>> particularly if there are topological relationships between different
>> layers. If a facility is related topologically to a road network, and
>> the point is shifted a few meters, this may result in disturbance of
>> the topology between these layers, rendering DHIS useless as a
>> repository. ogr2ogr is perfectly OK as long as we are not dealing with
>> these types of layers, but as soon as we start to think about
>> relationships to other layers, we need to be very careful about how
>> the data is preprocessed.
>
> Would you suggest then that the best place to clip precision would be
> when the data is retrieved from the database for the specific view/map
> rendering, rather than prior to it being stored?
>
> This would render the current convenience of storing as a geojson
> string redundant as we would need to process the string on checkout
> anyway.
>
> Can anyone say what the precision is on the shapefiles prior to
> ogr2ogr conversion  ie. are we introducing a new level of precision
> here or is that 15 digit precision the precision of the source
> shapefiles?

Quoting myself:

"Here is a comparison of what I get in GeoJSON vs GML (converting from the same
shapefile):
GeoJSON: 38.415412, 1.750212
GML:        38.415411724082148,1.750212388592194"

Both using ogr2ogr. So 6 vs 15 decimals.

Knut

> Bob
>
>>
>>
>>> We should be very conscient of not pushing the new, very simple
>>> solution too far, for more complex functionality we should rather
>>> employ Geoserver and PostGIS - and I still think this is the best
>>> solution for a national repository. Our new way of storing orgunit
>>> boundaries is a very small subset of such a full blown GIS solution,
>>> but has the advantage of being simple, lightweight and portable.
>>
>> Agreed on both points, namely that the solution is lightweight and
>> aimed at thematic mapping but other solutions would be more
>> appropriate for use as a repository of GIS data.
>>
>> Regards,
>> Jason
>>
>>
>>
>> --
>> Jason P. Pickering
>> email: jason.p.pickering@xxxxxxxxx
>> tel:+17069260025
>>
>



-- 
Cheers,
Knut Staring

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