Can there be an illuminance map outside of a building?

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You've tagged your question with "illuminance map" (an EnergyPlus construct), but also with "Radiance". So if you're using purely the former tool, then Dan's answer fully applies. If you're using Radiance only, there are no limitations on where you place your calculation points. If you're using OpenStudio and the Radiance Measure, you can get what you're looking for, somewhat, but it's pretty constrained:

If you have shading controls on your windows, the Radiance measure will place a calculation point on the largest window in each window group that has a shading control on it. The measure will calculate an 8760 daylight illuminance schedule for each of these points, as it's what is used for the shade control algorithm. This schedule is reported out in the Radiance output directory. It ain't much, but it's something. You'd have vertical daylight illuminance for all ortientations with a shade control. If you create Illuminance maps that exceed the space/zone boundaries, they will be exported to Radiance, but they will cause errors in the linked EnergyPlus simulation that the Radiance measure requires. Kinda makes an argument for a Radiance Measure option that simply generates illuminance schedules, doesn't it? =|

I don't believe you can do this with illuminance map. However, there are output variables to measure solar insolation:

Output:Variable,*,Site Solar Azimuth Angle,hourly; !- Zone Average [deg]
Output:Variable,*,Site Solar Altitude Angle,hourly; !- Zone Average [deg]
Output:Variable,*,Site Solar Hour Angle,hourly; !- Zone Average [deg]
Output:Variable,*,Site Diffuse Solar Radiation Rate per Area,hourly; !- Zone Average [W/m2]
Output:Variable,*,Site Direct Solar Radiation Rate per Area,hourly; !- Zone Average [W/m2]
Output:Variable,*,Site Exterior Horizontal Sky Illuminance,hourly; !- Zone Average [lux]
Output:Variable,*,Site Exterior Horizontal Beam Illuminance,hourly; !- Zone Average [lux]

You can read about these variables here.

If you need insolation on a particular surface you might try putting in a solar cell with 100% efficiency to measure solar input.