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?

[MikeStrouts]

Oh yeah, I forgot to mention that it takes 7-zip to open up the tz folder in the national atlas site.

[MikeStrouts]

Ok, so I THINK I figured out how to fix this.  The coordinate system in the National Atlas roads shapefile was already projected when I got it I think.  So I just redefined the coordinate system to a geographic one (WGS 1984) and then reprojected that file to the Maryland Harn 1983 one.  Now they all line up great .  I am wondering though if this was just a quick fix that could cause problems later or if this was really all I needed to do finally.

[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