Map scale is one of the most important factors to consider when representing data on a map. It is important not to lose the message of the map by including too much data.
The standard way of controlling the scalable visibility of data is by either using separate datasets of appropriate resolutions or by using layer properties in ArcGIS for each layer or group layer. However, this method is limited because 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 provides much more flexibility over what data to display.
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 big.
A dataset containing 100 shot interval point navigation for seismic data collected in the Potomac River acts as a good example for displaying a set number of data points to provide necessary data to view at an appropriate scale.
The image above shows how the quantity of data is not appropriate for the map scale. This makes the point data appear as linear objects. A better way of displaying this data is to show a subset of the data points representing every nth.
Queries in ArcGIS Pro still use VBA but how to access them has changed slightly. You can find access definition queries either right-clicking the layer or by selecting the layer and navigating to the Data tab and creating a Definition Query.
Interesting note ArcGIS Pro allows for the use of several definition queries, visible in a drop down. This could be used to quickly display different subsets of a database.
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.
The Definition Query
The modulo (described as MOD in VB) operation finds the remainder of the division of one number by another.
MOD(“ATTRIBUTE”, N)=0 will display every Nth point
can be used to reduce the number of points displayed for example MOD(“FID”, 10)=0 will display every 10th point from the FID attribute
To write the above expression we need to select the correct method to directly write the expression.
This definition query has defined that every 10th point is 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.
On a side note…
The other route to create a definition query is to use the query builder, using “Add Clause” within the software dialogue. Have you used this feature yet?
U.S. Geological Survey, Coastal and Marine Geology Program. Point Shapefile of 100 Shot Interval Point Navigation For Seismic Data Collected in the Potomac River/Chesapeake Bay from Sept. 6, 2006 to Sept. 8, 2006 on USGS Cruise 06018. Available from https://pubs.usgs.gov/of/2009/1151/data/navigation/seismics/shot100sort_geogmeta.htm