[TZGISUG] Tanzania Polygon

Wed Apr 16 13:34:06 EAT 2014

Hi Mike - missed the water body shapefile first time around!

Good and worthwhile assessment of the quality of the shapefiles. I was initially really excited, thinking we had a definitive set of boundaries for Tanzania. You're right, this is of real concern if the errors continue down to the ward level!

This is another good example of why we need a national SDI programme. Is anyone aware of any advances in NSDI? The last news article I can find is from 2011: http://www.ippmedia.com/frontend/index.php?l=28226

There is a 2007 concept document: http://www.topocarto.co.tz/downloadfiles/survey/DRAFT%20NATIONAL%20SPATIAL%20DATA%20INFRASTURE-3.pdf

The Survey and Mapping Division should theoretically be the custodian of definitive boundaries - does anyone know if they can provide digital versions of their basic admin data?

Cheers - Howard

On 16 April, 2014 at 12:36:06, Mike Shand (mike.shand at glasgow.ac.uk) wrote:

Hi Howard,

The Download zip file from NBS has 4 shapefiles - region; district; international bdy; and water body.

My observations on the shapefiles are that:
(a) The Water Body and the Districts shapefiles fit to each other.
(b) The Regions and the International Boundary shapefiles fit to each other.
There is a 250-300m shift between (a) and (b).
It does not appear to be a simple projection shift.
This then poses the question - which pair of shapefiles are correct - I have overlaid them all on Google and OpenStreetMap and cannot say which if any are correct.

My observations regarding inconsistencies with the coastline and lakes are that:
(1) Region and District shapefiles follow the lakeshore for Victoria but the lake centreline for Tanganyika and Nyasa.  For statistical mapping purposes the boundaries would normally follow the edge of water bodies.
(2) the northern lakeshore for Nyasa follows the coastline but the southern boundary follows the centreline.
(3) There is also an error in the most northern section of Lake Nyasa on the Malawi side.
(4) I have concerns about the interpretation of the coastline around the Rufiji delta, Kilwa and Mafia with regards to the alignment with the high/low water mark and tidal areas.
(5) on the Water Body shapefile a number of large and small islands are missing in Lake Victoria  e.g. Ukerewe
(6) there inconsistencies in dealing with internal lakes and administrative boundaries e.g. Lake Rukwa and Lake Natron, Kagera and their delineation.  Some small lakes are included some are not.
(7) there is a strange spike on the coastline south of Mtwara.

I think the NBS and/or Survey & Mapping Division should work together and take responsibility for addressing these issues which are major problems for GIS users.  If we as users all take it upon ourselves to fix these issues independently we will have a real problem resulting in many differing datasets circulating around the users of Tanzania GIS data.

It is projected that the Ward and Village boundaries will be released by NBS this year, if these issues persist with these datasets then as users we will continue to have a real problem.   Do we have any subscribers from NBS or SMD that could take forward these data quality issues.

Regards,
Mike

************************************
Mike Shand,
FRSGS; MAppSci; FBCartS,
Honorary Research Fellow
(GIS Cartographer / Consultant)
Geographical & Earth Sciences,
University of Glasgow,
Scotland, U.K. 
Tel: +44 (0)141 330 4780
E-mail:  Mike.Shand at glasgow.ac.uk

Hon. Secretary:  Society of Cartographers  http://www.soc.org.uk/
HOME PAGE & TANZANIA Links:  http://www.gla.ac.uk/schools/ges/staff/mikeshand/
************************************
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