I'm making a series of maps illustrating different aspects (road density, location of dams, elevation etc...) of a single watershed in California. In most of my maps, I've had the projection set to the geographic coordinate system, NAD_1983 (I know it's better to use a projected coordinate system but some of my data layers for some of my maps just don't line up properly in the projected coordinate system and I don't have enough time or experience with GIS to figure out that problem).

However, I'm trying to make a map of road density, using the line density tool, and because I want my density units to be in km per square kilometer, I need to put my 'roads' layer in a projected coordinate system (otherwise, I can only use square map units). If I change all of my layers to a projected coordinate system (NAD_1983_California_Teale_Albers), the map itself looks great BUT the scale bar is totally off (in the geographic coordinate system, the scale is about 2in=20miles versus 2in=0.00007miles for the projected coordinate system). Everything I can find online talks about the scale bar being off when using a Geographic Coordinate system rather than a projection- which I understand-, but not when using a projected coordinate system instead of a geographic one which is my problem! I think the issue with the scale is also impacting my density maps because my density values are completely off (they should be maxing out at about 10km of road per square km and instead I'm getting values as high as 80km/km2).

Does anyone have advice either on how to fix my scale bar and length calculations when in the projection or even how to calculate density in metric units rather than map units in a geographic coordinate system?

I am using one of the most up to date version of ArcMap.

  • My guess: you used define projection or the data property pages to change the coordinate system. You need to use the Project or Project Raster tools to get versions in Teale Albers. – mkennedy Nov 11 '15 at 23:15

You can get into a lot of trouble using projections and transformations, so be wary not to mess up your data.
If you're trying to get a geographic-coordinate system based shapefile into a projected coordinate system be sure to use the Project tool in the Data Management Toolbox (be sure to check this link as it has a lot of important info). Also, be sure not to define projection of anything you don't know the actual coordinate system.

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.