Using the binding.irb tool mentioned by @mechnicov, I was able to determine that instead of routing to the new method, I was getting an unauthorized viewer message. Turns out there was some legacy authorization code that I needed to account for in my tests.

Specifically, I added the pry gem to my Gemfile, updated the test to this:

describe 'GET #new' do
  it 'simplified test' do
    get :new
    binding.pry
    expect(assigns(:map_id)).to eq(1)
  end
end

Then I ran the test, and examined the response object (which is what this controller is for), then investigated its contents.