Marshall Proposed Trail System

We describe steps we followed to overlay a trail map over the street maps we used to reach trails.

LINEUP OVERLAY 45.4446792,-122.7015352 45.4589623,-122.6851040 BOUNDARY 45.4446792,-122.7015352 BOUNDARY 45.4589623,-122.6851040 Trail Map Overlay

We found a detailed and forward looking map of Marshal Park and related natural areas in the Habitat Management & Trail Plan, Marshall Park Natural Areas, June 2009. pdf

# Extraction

Remote image

We preferred the map found on page 27 titled "Proposed Trail System For Marshall Park & Foley Balmer Natural Area". We extracted this single page as a print preview, cropped the map, and then saved this as an image file.

This might have been sufficient for an overlay had the map not been rotated from a north-up orientation.

We restored the required orientation by placing the image on a blank Keynote slide and then rotating this until the street grid aligned with a horizontal construction like I added for the purpose. I exported this as image per slide.

I returned to Preview where I again cropped and sized the map in its new orientation. I erased the introduced margins with Instant Alpha and saved this as the overlay image.

# Placement

Remote image

The Map OVERLAY markup requires the lat/lon for the lower-left and upper-right corners of the image. I estimated and marked these by imagining verticals and horizontals from the tilted corners I could locate on the map. I was close enough that I could see top, bottom, left and right placement errors.

I canceled out these errors one at a time by finding a map feature, zooming until the error was half a map width or height, then planting a new corner marker correspondingly displaced and using digits from this in the overlay markup.