Georeferencing is the process of assigning real-world coordinates to a scanned map, aerial photograph, or drawing that started life as a purely pictorial document. Once georeferenced, the image lines up with modern base maps, GPS data, and feature services — letting you overlay nineteenth- and twentieth-century documents on today's imagery.
A geodetic datum defines the reference ellipsoid and origin that latitude and longitude are measured against. When the datum changes, every point's coordinates change too — even though the point on the ground has not moved.
| Aspect | NAD27 | NAD83 |
|---|---|---|
| Year | 1927 | 1983 (realized 1986) |
| Ellipsoid | Clarke 1866 | GRS 1980 |
| Origin | Meades Ranch, Kansas (ground-based) | Earth's centre of mass (geocentric) |
| Typical shift from NAD27 | - | ~20 to 100 metres (varies by region) |
| Used by | Legacy USGS topos, older ROW plans | Modern ArcGIS, GPS, federal surveys |
Opens a floating panel with the complete layer list from the web map. Toggle the ten ROW survey sheets on or off individually, and turn the NAD27 and NAD83 markers on or off to compare them side by side. A small inline legend is available under each layer.
A dedicated legend panel showing the symbology for every currently visible layer. Use this when you want the full symbology in one place rather than inline under each layer.
The web map ships with eleven saved extents: the full-coverage Survey Sheets (All) bookmark plus one for each individual ROW sheet 1 through 10. Click any bookmark to pan and zoom the map to that extent.
Swap between World Imagery, Topographic, Streets, Navigation, Light / Dark Gray, and other Esri-curated basemaps. The ROW surveys and markers stay pinned on top of whichever basemap you choose.
The Zoom and Home buttons in the top-left corner handle standard navigation — Home returns the map to the Ogunquit Sheet 1 detail extent the app opens on. The Scale & Extent button in the bottom-left opens a dialog with the current scale (as a ratio and as 1” = N′), zoom level, and the four WGS84 decimal-degree corner coordinates of the visible extent. Each value has a Copy button for quick sharing.
These are the same survey-marker point files that drive the SM NAD27 and SM NAD83 layers on the map. Each row carries the marker's Northing and Easting in Maine State Plane West coordinates, plus the corresponding Longitude / Latitude (decimal degrees and degrees–minutes–seconds) in its own datum.
Download either file below and open it in ArcGIS Pro, QGIS, Excel, or any geocoder that accepts XY input to reproduce the shift you see on the map. Plot the NAD27 file against a NAD83 basemap and the ~40–60 metre offset is the datum shift itself.
The four reference images below show the source survey documents these markers were extracted from — a December 1997 Right-of-Way survey of Ocean Avenue, Town of Wells, Maine, prepared by Dow & Coulombe, Inc.
