Scale is one of the most important factors to consider when representing data on a map. As mentioned in an Exprodat blog on Creating a Multi-scale Geological Map it is important not to lose the message of the map by including too much data.
The standard way of controlling scalable visibility of data is by either using separate datasets of appropriate resolutions or by using the layer properties in ArcMap for each layer or group layer. However, this method is limited, when viewing data at a layers’ visible scale all the data is either all on or all off. This may not be appropriate for the type of data being viewed.
Using definition queries gives us more flexibility over what data we want to view. For more information on definition querys and displaying subsets of data through this method have a look at this ArcGIS help page.
Lots of data collection points!
When gathering data in a seismic acquisition, receivers (geophones) are towed (when in water) or placed at specific intervals (when on land) and are recorded as a data point. Additionally, a shot point is also usually recorded to be viewed in a spatial context along the seismic line.
These data points are located at regular intervals and depending on the length of the line the number of points can be enormous.
A 634 km seismic line that that forms a part of the Gawler-Officer-Musgrave-Amadeus (GOMA) Deep Crustal Seismic survey was acquired by Geoscience Australia (GA) and acts as a good example for displaying a set number of data points to provide necessary data to view at set scales.
The final dataset contains 31,587 points that represent the shot point and are found equally spaced along the seismic line at 20 meter intervals.
Figure 1 View of whole data extent
The image above shows how the sheer quantity of data makes the collection of 31,000+ points appear as one linear object. A better way of displaying this data is to show a subset of the data points representing every 1,000th point or so.
After zooming into an area around Mount Willoughby, South Australia to a scale of 1:250,000 the original dataset still contains so many data points that the data appears to be a single linear object.
Figure 2 View of data without definition query applied
Figure 3 View of data with definition applied
Using a definition query it is possible to show every nth data point. This provides the flexibility to show the number of data points we feel is adequate for visualising the data at the map scale.
If needed this definition query can be used on several copies of a layer combined with the standard scale tools as mentioned previously.
The Definition Query
The modulo (described as Mod in VB) operation finds the remainder of division of one number by another.
The statement is:
Mod(ATTRIBUTEVALUE, N)=0 will display every Nth point
For example: Mod(OBJECTID, 50)=0 will display every 50th point from the OBJECTID attribute
This definition query has defined that every 50th point be viewed in the layer. For cartographic purposes this definition query allows the user to select an appropriate display of data for the scale in use.
Figure 4 View of dataset with every 50th point being displayed through use of the definition query
Maher, J. L., (2008) L190 Gawler-Officer-Musgrave-Amadeus Deep Crustal Seismic Survey, SA & NT, 2008. Stacked and Migrated Data and Images for 08GA-OM1. Available from http://www.ga.gov.au/metadata-gateway/metadata/record/gcat_70579