topic Re: Population Density with graduated colors isn't reflected in map in ArcGIS Spatial Analyst Questions

Population Density with graduated colors isn't reflected in map

France — Fri, 01 Feb 2019 06:06:19 GMT

Hi,

I'm using ArcMap 10.3.1, 180-day trial version, to do a population density per square mile map with graduated colors but instead of getting a map with different colors for the different classes, my entire map is all one color and it's the color for the lowest value class. I checked the histogram and there are values for the other break points but it's not reflected in the map.

Here's what I did:

Using Census Block Group data, I added a column to the layer's attribute table, called it "SqMiles" and set the type to Float.
On the attribute table, for the new column, I right-clicked and selected "Calculate Geometry", set Property to "Area" and Units to "Square Miles".
For Layer Properties, Symbology tab, Quantities > Graduated Colors, I set the value field to "Population 2010" and Normalization field to "SqMiles" (the column that Calculate Geometry populated).

Attached are my results. The map should have multiple colors but instead has just one color. If anyone has the "GISTutorial 2" book by David Allen, I'm following the directions on pages 110 - 113.

I tried checking online for solutions, zooming the map, adding the SqMiles column data type as Double, changing the number of classes, clearing ArcMap's cache, restarting my laptop, checking with ESRI that my trial license covers Spatial Analyst (it does) but I'm still getting all yellow instead of different colors for the different population densities. Any suggestions?

Many thanks!

Re: Population Density with graduated colors isn't reflected in map

StephenM — Sat, 02 Feb 2019 01:19:21 GMT

Those population density numbers look off. Maybe the area didn't calculate correctly. What do your population and area numbers look like?

Re: Population Density with graduated colors isn't reflected in map

France — Tue, 05 Feb 2019 00:21:48 GMT

Hi,

Forgive my delay in responding. I had family in town this past weekend.

I used ArcMap's "Calculate Geometry" feature so it calculated the area for me. Attached are the first 22 rows of the table. Age and household columns are not displayed but the rest of the columns are displayed.

I don't know how ArcMap calculates Square Miles and what it uses for inputs from the attribute table. I just assumed that since I was using "Calculate Geometry" that it was calculating correctly. Many thanks for any suggestions or solutions you can offer.

Re: Population Density with graduated colors isn't reflected in map

StephenM — Tue, 05 Feb 2019 02:32:13 GMT

That's the correct way to calculate geometry. Those areas definitely look too small though. Block group 481210202024 (the top one in your attribute table) looks like it should be around 48.26 square miles (based on Esri's USA Block Groups ArcGIS Online layer).

You might try making sure the projections are all set up correctly. Someone in this thread had a similar issue:

Calculating area using Calculate Geometry

Re: Population Density with graduated colors isn't reflected in map

DanPatterson_Retired — Tue, 05 Feb 2019 02:48:46 GMT

I think your geometry is in decimal degrees

You can either project the file to a projected coordinate system, then calculate the values or

Calculate Geometry Attributes—Data Management toolbox | ArcGIS Desktop

you can calculate geometry in a projected coordinate system, IF you specify one (ie like a suitable UTM or Albers)

Re: Population Density with graduated colors isn't reflected in map

France — Tue, 05 Feb 2019 03:39:05 GMT

To follow along with using the top row in the attribute table, block group 481210202024, I selected "Identify" and flashed it, found the block group on the map and zoomed in. It's a little sliver and used the measure tool to measure the length (north-south direction) and came up with 0.489728 Miles (I don't know what happened to the second toolbar with the "insert pictures" command on the reply editor window).

The projection is State Plane NAD 1983 and the coordinates are in feet.

Data Frame Properties - NAD_1983_StatePlane_Texas_North_Central_FIPS_4202_Feet

Layer Properties > Source - Feature Class - CensusBlkGrp; Geographic Coordinate System - GCS_North_American_1983; Datum - D_North_American_1983

It looks like it's set up correctly.

Re: Population Density with graduated colors isn't reflected in map

DanPatterson_Retired — Tue, 05 Feb 2019 03:57:28 GMT

CensusBlkGrp; Geographic Coordinate System - GCS_North_American_1983; Datum - D_North_American_1983 has the units decimal degrees, you either need to project the file or follow the procedure to get area units into a projected coordinate system (in a new column), then calculate density from there

Re: Population Density with graduated colors isn't reflected in map

France — Tue, 05 Feb 2019 03:59:49 GMT

Oh okay, got it. I'll try that. Many thanks!

Re: Population Density with graduated colors isn't reflected in map

France — Tue, 05 Feb 2019 04:09:44 GMT

Hi Stephen, thanks for your help. I'll try Dan's solution.