How does ArcMap know where to draw objects?

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10-27-2011 12:14 PM
ChristopherMinniti
New Contributor
This is a decidely insignificant question but I've been curious for as long as I've used ArcGIS where the program retains the data/information for the particular geometry of objects.  And by this, I mean where is the information retained about the particulars of the polygons, such as number and relative location of verticies, length of each segment comprising a polygon, etc.

For the sake of clarity (and because I don't feel I've expressed my question well), this question comes from a discussion I was having with my co-worker.  Unfortunately, I am the resident GIS expert at my company, which is disconcerting since I'm no where near an expert with GIS.  As the "expert" I was asked by my co-worker if it is possible to fill out a table in excel with all the information necessary to create a shapefile before ever entering or using ArcMap?  I had no idea what to say...
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JimW1
by
Occasional Contributor II
This is a decidely insignificant question but I've been curious for as long as I've used ArcGIS where the program retains the data/information for the particular geometry of objects.  And by this, I mean where is the information retained about the particulars of the polygons, such as number and relative location of verticies, length of each segment comprising a polygon, etc.

For the sake of clarity (and because I don't feel I've expressed my question well), this question comes from a discussion I was having with my co-worker.  Unfortunately, I am the resident GIS expert at my company, which is disconcerting since I'm no where near an expert with GIS.  As the "expert" I was asked by my co-worker if it is possible to fill out a table in excel with all the information necessary to create a shapefile before ever entering or using ArcMap?  I had no idea what to say...


From a hack's perspective:

The geometry of features are stored in binary fields or binary file formats:

Personal geodatabase - it is stored in the [Shape] field as an 'OLE Object' which means it is some sort of binary specific to ESRI. If you Google 'ESRI Ole Object' and know some VBA you'll get a good idea of what is going on. This is by-far the easiest format to learn what Arc is doing when it stores your data.

In SDE (Sql Server) it's stored as a varbinary field that is linked to the feature class through a couple of tables. I've never bothered trying to join the correct tables to get a full view but it is basically a more complicated mdb structure to allow for versioning and such. I leave the technical stuff on SDE to the DBAs.

Shapefiles - proprietary ESRI binary format stored in the .shp portion of the shapefile - the table is in the .dbf and the .shx and .sbn are secret handshakes

File Geodatabases - a hybrid between shapefiles, personal SDE, and old school ArcInfo coverages but I'm still convinced it's just a repackaged ArcInfo coverage with SDE conventions :)

This is why you need VBA or Python to make any changes to the geometry of a feature class.
So it is, technically, possible to fill out vertices (say COGO) in Excel and through VBA make feature classes. I have had success with MS-Access databases where a user enters a new point in Access and the back-end changes the feature classes' geometry using VBA in Access.

I'd say 90% of GIS users will never ever touch the back-end of these data storage formats so I wouldn't worry about being the expert and not knowing. I've worked with many extremely talented GIS professionals that wouldn't have the first clue how to edit the properties of a feature through code.
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GregoryElwood
New Contributor II
"...I leave the technical stuff on SDE to the DBAs."

I must be extra thick this morning...I can't figure out what "DBAs" means.

Doing Business As...???

Dead Broke Artists???

Darn Building-block Arrangers???

Of course, it'll be something completely simple, and I'll look (and feel) like a idiot.

Happy Halloween everybody!

Gregory
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JimW1
by
Occasional Contributor II
Haha - those pesky Database Administrators (DBA). They get mad when lowly GIS Analysts like me try to make suggestions for SDE or SQL Server.
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DarrenWiens2
MVP Honored Contributor
If you follow this paper closely, it tells you how you could literally type out a shapefile, byte by byte. Have fun! :rolleyes:
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JimW1
by
Occasional Contributor II
If you follow this paper closely, it tells you how you could literally type out a shapefile, byte by byte. Have fun! :rolleyes:


I completely forgot about that white paper!
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