Module 10: Dot Mapping

This week was Dot Mapping.  Dot maps indicate locations of an occurrence (in this case conceptual points), where each dot represents a set amount of that occurrence.  Consideration should be taken with enumeration unit (small unit best), dot value (trial & error, nomograph or software) and dot size.  The dots can be placed:  1)uniformly in the enumeration unit (not preferred: impression of continuity across the area), 2)geographically weighted, those of higher values are weighted to be closer to other higher values or 3)geographically based, utilizing ancillary information both limiting attributes and related attributes to place dots based on area attributes.  Data criteria for Dot Density is conceptual data (raw counts), discrete, used to compare or portray variations or patterns. 

There are advantages of Dot Density maps.  Dot Density maps are an easy concept to understand.  They are effective at portraying variations.  Ideally the dots could be counted to recover data.  These maps can be adapted to include other phenomena: urban areas, slope, bodies of water.  Dot Density can be used in correlation with other types of maps. 

There are also disadvantages of Dot Density mapping.  The map can be hard to estimate density.  The map reader could misinterpret a dot as a single occurrence.  Computer dot placement can misrepresent patterns or lack of patterns.  Possible additions to counteract some disadvantages could include a clear legend to avoid the dot as a single occurrence and utilizing ancillary data to avoid random placement.

This week's lab was to represent the 2000 population density of South Florida in a dot map.  Provided in the lab materials were shape files of South Florida, Surface water and urban land use as well as table with census population data.  

The tabular data was joined to the south florida shape file (with both shape file and tabular data (utilizing add data button) displayed in ArcGIS>right click south florida layer>join and relates>join>based on county name, joined based on area).  Dot Density symbology was accessed through the south florida layer properties under the symbology tab (quantities, dot density. population was selected and added as symbol).  Dot size and dot value (1-5) were adjusted until max preview started to coalescence.  For my specific map I utilized a dot size of 4.5 and a dot value of 10,000.  I adjusted the color (several times) to find a color that would stand out and clearly be the highest in the visual hierarchy.  I finally, chose to utilized a bright full purple.  The counties included in the south florida shape file were adjusted and examined with different colors and outlines and without. "Maintain Density by" feature was turned off so the dot value or dot size would not change when zoom in and out.  Properties window within symbology tab was opened and the dots were set to fixed placement, this prevented movement when ArcGIS redrew as changes were made.  Finally, masking was also specified in the properties window.  Initially the mask was set to exclude the surface water layer.  Although this did keep the population points from being located in a body of water, the arrangement of the dots density still had clear differentiation at county boundaries.  A second mask option was applied that placed dots within urban land.  This option presented the dots clustered in and around urban land, more as it would be in the natural world.  The rest of the map was adding essential map elements (title, legend, north arrow, credits, projection,author, date).  Masking the dots really bogs down ArcGIS, so the mask was turned off while final elements were added and style choices made.  Lab instructions requested geographic reference by labeling some major cities in the area.  I choose cities that would be large enough to be recognized.  I added a short integer field to the Major Cities (obtained shape file from previous lab) attribute table named display.  I started editing to input for that field.  I utilized "1" for those cities I wanted displayed and "0" for those I did not want to display.  I stopped editing and saved my edits.  In the symbology tab, category unique feature, value field set to my new field "display", turned off "all other values", "add all value" button, chose symbol for "1" and highlighted the "0" option and clicked remove button, ok.  Due to the placement of the city names amongst the dots I added text for the labels.  I left Tampa's label covered up by dots.  I could not find a place where it would work.  I differentiated the surface water types and added a legend for that layer.  The typical legend by ArcGIS, created for the population layer, only provided the most basic information, one dot = value.  Three visual anchors showing low, medium and high densities. I drew these features in ArcGIS.  I drew a box and copied and pasted it twice to ensure all three where the same size.  I aligned them with the align function.  I added a dot (rectangle drop down box and chose marker), adjust the dot to the same size and color as I utilized in the symbology, and copied and pasted the desired amount.  I symbolized low density with 3 dots (30,000 people), medium density with 20 dots (200,000 people) and high density with 50 dots (500,000 people).  The layer organization I decided to leave the layer with the dot density hollow - without fill and without outline.  I set a second florida shape file as the lowest layer with a bright green for the background.  The surface water layer and major city symbols were in the middle and finally the dot density layer with the urban mask turned back on is the top layer.