Friday, 8 April 2011

Project Evaluation - ArcGIS Data Exchange Issues

We've hit a problem in exchanging data from the HALOGEN databases with ArcGIS. Up until now we've been exporting the data from the databases into files as tab separated fields, importing into ArcGIS and converting into shapefile data from there. However ArcGIS's shapefile format (or more specifically its associated .dbf database file) has some severe limitations which mean we can't realistically continue down this route.

ESRI has this to say about the use of shapefiles:

With some exceptions that are noted below, shapefiles are acceptable for storing simple feature geometry. However, shapefiles have serious problems with attributes. For example, they cannot store null values, they round up numbers, they have poor support for Unicode character strings, they do not allow field names longer than 10 characters, and they cannot store both a date and time in a field. These are just the main issues. Additionally, they do not support capabilities found in geodatabases such as domains and subtypes. So unless you have very simple attributes and no geodatabase capabilities, do not use shapefiles.

(see Geoprocessing Considerations for Shapefile Output).

Of these limitations, I believe the inability to represent NULL values is the most serious - NULLs are represented as zero within the shapefile's .dbf format, and this means that any value of zero within the data cannot be trusted at all.

I am looking at the ArcGIS Data Interoperability extension so that we can plug ArcGIS directly into the HALOGEN database and avoid these problems. Unfortunately there's no fully-featured solution for connecting ArcGIS into MySQL, so for evaluating this I'll be porting the HALOGEN databases to PostgreSQL.


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