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Density map and scale bar problems

MikeStrouts

So, I'm trying to create a kernel density map with some point data I geocoded from some FARS tables on vehicle accidents around the DC beltway area but I'm having trouble getting the map units and scale bar to work at the same time. I first tried leaving the shapefile as it was originally projected in the GCS_North_American_1983 CRS but I couldn't get the map units in the kernel densisty dialogue box to show anything in km, m, miles, etc. It just showed up as map_units and I couldn't change anything. Then I tried defining the projection as maryland state plane, which let me use the correct map units of meter that I wanted but when I went to put a scale bar in the main map the numbers were way off. It was showing the whole area of DC as like .0005 miles or something crazy like that? Then I tried using the project tool to see if that would change anything but the scale bar was still weird. What am I doing wrong?

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IanMurray

Hi Mike,

It sounds like you need to change the projection of your dataframe or your data frame units, not that of your data. If you shapefile comes projected, then define projection would mess up its location and scale, define projection is only to be used when you have you have an unknown or incorrect coordinate system defined. If you need to project your data in a new projection, use the Project Tool.

Your dataframe coordinate system and map units are defined by the first layer you add to your map, in this case GCS_North_American_1983 CRS from your shapefile, which the GCS_North_American_1983 GCS using degrees as its angular unit, hence your map unit being degrees. Try changing your dataframe coordinate system to one that uses feet as a unit., by going to View > Dataframe Properties> Coordinate Systems, Maryland state plane does use feet so that should work fine. You can check your map units under Data frame Properties > General tab, and can also change what the map display shows for units there.

I would use define projection to put your data back in the correct projection, then change your dataframe coordinate system to Maryland State PLane. THere is no need to reproject the data into Maryland State Plane unless you want to, since ArcGIS projects data in a different coordinate system than that of the data frame on the fly, so it does it for you for display purposes.

I hope this helps

Dan Patterson‌, Melita Kennedy‌, got any other good advice for Mike here?

DanPatterson_Retired

To follow up on Ian's comments...I always take the extra step to Project (aka the Project tool) to get my data into a projected coordinate system when I am going to deal with any data set that uses any part of geometry (ie. length, area, volume, position and function that rely on them). I have never liked, nor do I trust, projections on the fly, since, they make people complacent about what they are doing...MTCW

TimothyHales

I've moved your post into the Mapping space. You will get a much better answer here as the GeoNet Help is intended for community help and feedback. You can see more on the community structure, and what topics are under each space from the following documents:

GeoNet Community Structure

ArcGIS Discussion Forums Migration Strategy

Thanks!

Timothy

MikeStrouts

Hi Ian,

I tried doing that but it didn't work. My scale bar still read .0005 miles for the width of Virginia. Would having my shapefiles/feature classes in a geodatabase be part of the problem? I did eventually get something to work but only after I exported one of my feature classes to a folder in explorer and reprojected it to NAD_1983_HARN_StatePlane_Maryland_FIPS_1900_Feet. When I made a new arcmap document and set it as the first layer everything was fine.

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IanMurray

Did you define projection of your data back to your original projection of GCS_North_American_1983 CRS, then change the dataframe projection to NAD_1983_HARN_StatePlane_Maryland_FIPS_1900_Feet?

The reason what you did worked is after you projected the data into a new projection and added it to a new map document, the map document automatically set to that coordinate system and projection, which happened to have a units of feet, which is what you needed. Being within a geodatabase should have had nothing to do with it.

MikeStrouts

I just tried that and it got my scale bar working like normal again but now I can't change my area units when I try to do a kernel density analysis.

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