Thursday, November 27, 2008

Subtypes and Domains. What is what?

Subtypes, range domains and coded value domains. These are all terms that you may have heard tossed around in the ESRI world. But what exactly are they, and when should you use them? Here is a brief overview of the different terms.

Domains are rules that are applied to individual attributes that define the possible values for that property. There are two types of Domains in a Geodatabase: Range domains and Coded Value Domains.

Range Domains, as the name suggests, enforce a valid range of numerical values for an attribute. For example, an Age attribute for a Person feature class could be restricted from 0 to 100, meaning that all values between 0 and 100 are valid.

Coded Value Domains enforce a fixed list of values on a field, on any text, numeric or date field. These can be thought of as a look up table, as you can have a different code (the value that is actually stored in the field) to the description (the value that is displayed to the user when entering data). For example, a cadastral parcels feature class may have a zone property defining whether the property is residential (zone 1), commercial (zone 2) or industrial (zone 3). Coded value domains allow you to store the zone number, but display the more meaningful description to the users.

Subtypes are a sub-category of features in a feature class, that all share common properties (attributes). For example, you maye have a feature class of Animals, which you wish to categorise into species. In this case, the species would be the subtype.

The advantage of using subtypes is that you can specify different domains based on the subtype. Using the animals example again, you may have a height attribute with an associated range domain. You would most probably want a different range domain for elephants than for mice. Subtypes allow you to do this.

Subtypes and Domains in ArcPad
The good news is that since the release of ArcPad 7.1 and the new AXF file format, ArcPad directly supports subtypes and domains. This way all of your Geodatabase rules can be enforced at the time of collection in the field, making the data migration process from the field back to the office all the more easier.

GIS Acronyms & Dictionary

GIS makes frequent use of a range of terms and acronyms.

There are a number of online resources and GIS dictionaries which define these terms.

A few of these online resources are:

Monday, November 24, 2008

Don't forget the shapefiles!

Did you know that the drawing performance of shapefiles in ArcPad is much better than the new AXF format? The difference is particularly noticeable for layers with a large number of complex features, such as a cadastre or road network.

The table below shows a comparison of the drawing time for the Riverside Parcels sample dataset that comes with ArcPad 7.1 on a HP iPaq 2790. This layer has a total of 3800 parcels. All times are in seconds.

So for any background datasets that are not being edited in the field, you are far better off exporting them to shapefiles rather than AXF.

Shapefiles can easily be exported using the Get Data for ArcPad wizard in the ArcPad Data Manager tools for ArcMap. The one way arrow (check out only) makes a read only shapefile; the two way arrow (check out and in) creates an read and write AXF file.

Thursday, November 20, 2008

Map Scales - Which is Which

Maps can be an extremely useful resource in any field based mapping project.

Associated with a map is a map scale.
The scale is the ratio of a unit of distance on the map to the equivalent distance on the surface of the earth. (For example 1:25,000 means that 1 cm on the map represents 25,000 cm on the ground, or 250 m).
A map scale is usually represented as a Ratio (1:25,000), but it can also be represented as a Fraction (1/25,000).

Maps are referred to as either 'Large Scale' or 'Small Scale' maps, which tends to cause a fair bit of confusion. Which scale is which?

Large Scale:
  • The features represented on the map appear larger. (The map covers a smaller extent of the earth).
  • The map scale, when represented as a fraction, is larger. (For example 1/1000 is a larger fraction than 1/50,000).

Small Scale:

  • The features represented on the map appear smaller. (The map covers a larger extent of the earth).
  • The map scale, when represented as a fraction, is smaller. (For example 1/1,000,000 is a smaller fraction than 1/50,000).
It should be remembered that the use of the terms, Large and Small scale, are subjective, and are somewhat dependent on the map users perspective. The principle however remains the same.

Thursday, November 13, 2008

.GPX - GPS eXchange Format

The GPS eXchange Format is an XML based file format used to describe GPS data. GPX data can be seamlessly transferred between GPS’s and applications that support the format.

Further information about the GPX format can be found at:

The GPX format is supported by a range of applications, including:

GPS eXchange Format and ArcGIS

A range of third party applications, including those listed above, can be used to convert .GPX files either directly to shapefiles (or GeoDatabase feature classes in the case of FME), or to a text format which can then be converted and loaded.

In ArcGIS ArcMap:

Within ArcGIS the .GPX format is supported using the Safe FME ArcGIS Data Interoperability Extension. Information about the formats supported is available from both Safe Software and ESRI.

The ArcPhoto Extension, which can be downloaded from the ESRI ArcScripts site, includes support for the upload of GPX Data, from a GeoProcessing Model. Further information can be accessed from ESRI ArcScripts, and search for either 'GPX' or 'ArcPhoto'.

In ArcGIS Explorer:

ArcGIS Explorer has native support for the import of .GPX data from the Tools > Import Menu.

Furthermore, a custom task, the GPS Data Manager, can be accessed and downloaded from the ESRI ArcScripts site enables the upload of GPX Data. Further information can be accessed from ESRI ArcScripts, and search for 'GPX'.

In ArcPad:

Within ArcPad .GPX data converted to Shapefile format, or data that has been checked out into AXF format, can both be utilised.

Usually ArcPad is configured to communicate directly with a GPS receiver. The data is streamed using a specific GPS Protocol and the spatial features are captured using the GPS location from within ArcPad. Features are stored in either a Shapefile or AXF feature class, and can be synced (checked in) and converted from within ArcGIS.

Friday, November 7, 2008

Movember Madness

Hi All

Welcome to the RIA Mobile GIS Blog. Here we will keep you abrest of all the coming and goings in our world of Mobile GIS. But to start things off we decided sign up to Movember to help raise funds for men's health issues. Unfortunately we have no before photos but by the end we hope to some mighty moe's for you all to see.

If you would like to donate to this cause follow this link.

Movember - Sponsor Me