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I have some shapefiles that I am reading and then loading into tables in the database. The shapefiles with no zoom (eg. "au_postcode") have no shapes with type null. When I tried to load a shapefile with zoom (eg. "au_postcode_100"), I saw some null shapes.

It would be great if someone can please explain to me how the null shapes work and why would a shape file be null. Its kinda confusing because to me a shape is at least a point and if there is a physical location, there would be a shape for it.

I am new to GIS and if something in this question does not make sense or I've used the wrong terminology, please let me know. Thanks

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+1 Your question makes great sense. I still do not understand why most (all?) GISes do not distinguish between, say, null polygons (no information available at all), polygons with some locational information (such as a point), and polygons with some extent information (but no area). These are useful distinctions and they are possible to maintain in the shapefile format. –  whuber Jul 5 '12 at 2:03
    
Thanks, I guess the confusion is also how to display a null shape on the map. my guess is that it will just not show anything, but there is centre point X and Y, which add to my confusion. –  Divi Jul 5 '12 at 3:11
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1 Answer 1

up vote 3 down vote accepted

A shapefile (and many other file formats) can contain records in the table but no geometry. In a polygon file (or any shapefile), if you create a new row in the table, it doesn't automatically create a polygon. Yet, if you create a polygon (by digitizing) it will automatically create a record.

When you say there is a centre point, do you mean in the attribute table? If so, you will need to create the geometry yourself.

Hope that helps.

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Thanks for your reply. I understand that there can be a null shape/geometry. The question really is why would a record have that. By centre point, I mean the centre points X and Y for that row. –  Divi Jul 5 '12 at 3:34
    
There could be several reasons why a null record exists. Perhaps the rows were imported from a spreadsheet without the geometry being created yet. Someone may have also manually created the rows and hadn't drawn geometry. Often, these are mistakes and may need to be deleted. But, they could be valid. If you have the x/y, you can create a point from that (most likely a centroid if it is a polygon). –  Fezter Jul 5 '12 at 3:47
    
I guess its human error then. But to be very honest, it seems too unprofessional because these shape files were provided to us by a company that deals with GIS products. –  Divi Jul 6 '12 at 6:15
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