How to find the exact location of a site plan from its UTM coordinates and open it in Google Maps

This is what the plan showed, as it arrived:

Viewer coordinates belong to the point you tap in the drawing. On a layout, which is what the screenshot showed, they're millimeters of paper; in model space they're only UTM if the plan was drawn in real coordinates. Here, 266 and 106 match no place on the ground.

The location map grid helps you find your bearings in town, not measure. And in this case, as step 4 shows, it had a wrong digit.

The coordinate table, with the zone and datum from the title block, is the only thing that locates the site precisely.

UTM coordinates alone aren't enough. You need three things:

1. Easting and northing, in meters. In the southern hemisphere the northing has seven digits and the easting six.
2. Zone and hemisphere. The same easting repeats in every zone: pick the wrong zone and the point jumps hundreds of kilometers. The zone follows from the longitude: zone = 1 + floor((longitude + 180) / 6).
3. Datum. Pick the wrong one and the point moves tens or hundreds of meters.

In our case the title block said it all: zone 17, southern hemisphere, datum PSAD 56. In the US, NAD83 UTM coordinates plot correctly as WGS 84 within a few meters; NAD27 and State Plane coordinates need converting first.

Google Maps, phone GPS and Google Earth all work on WGS 84. Many older Peruvian plans are on PSAD56, the datum of the country's earlier mapping, still in circulation even though Peru adopted WGS 84 in 2006.

The same point has different coordinates on each datum. On our plan:

The gap is 447 m to the northeast: another block or another neighborhood. In UTM terms, going from PSAD56 to WGS 84 here subtracts about 252 m from the easting and 369 m from the northing.

A coordinate table can be checked against itself: the distances and angles it states must match the ones its coordinates give. On this plan we found three things.

The lot's table is consistent. All four sides and four angles match the coordinates, to the millimeter and to the second:

The lot encloses 94,076.58 m², about 9.41 ha.

The location map grid has a wrong digit. Its labels read 763,200, 763,600 and 764,000 for the easting, while the table says 773,xxx. That's 10 km: read as is, the grid points to downtown Chimbote. Which one is right? The table, because it lands among the neighborhoods the plan itself labels. The grid had a 6 where a 7 belonged.

The second table doesn't close. The plan has a second area, corners A to D, stating sides of 16 and 30 m, a rectangle. Its coordinates, though, draw sides of 16.07, 47.00, 33.07 and 30.42 m. One of the two is wrong, and before using that area you have to confirm it with whoever made the plan.

1. Open UTM coordinates on Google Maps.
2. Paste the coordinate table into the tool's table as it comes from the PDF, Excel or AutoCAD, with corners, sides, distances and angles: one Ctrl+V in any cell does it. The tool finds the easting and northing in each row, fills the columns and separates the tables. You can also type the points row by row; for a single point, use «One point».
3. Pick the plan's zone, hemisphere and datum; in the example, zone 17, south and PSAD56. If the plan doesn't state the datum, choose «I don't know» and compare where each reading lands.
4. Read the result: Your plan is here, with the latitude and longitude of the lot's center, the outline on the map and a dashed outline showing where you'd land without the datum shift.
5. Check the coordinate table check: what matches in green, what doesn't in amber.

Google Maps accepts coordinates in decimal degrees, degrees-minutes-seconds or decimal minutes. So the tool gives you four outputs:

• Open in Google Maps: the point, ready to save or share.
• Directions: a route from wherever you are.
• Satellite view: the aerial photo, to recognize the site before you go.
• KML: the lot outline for Google Earth.

How accurate is it? On WGS 84 or NAD83, as accurate as the plan's coordinates. On PSAD56, the datum shift is good to about ±16 m: enough to reach the site, not to stake boundaries. For that, tie the work to control points surveyed with GNSS.

If you also have the DWG and want the drawing to sit on a map:

1. Check what coordinates it was drawn in. In model space, not on a layout, tap a corner: if you don't see its easting and northing from the table, the plan is in local coordinates.
2. Move it to real coordinates with the ALIGN command: pick two corners in the drawing and give them their table coordinates. The plan moves, rotates and scales into place.
3. Assign the drawing's coordinate system. In Civil 3D, under Drawing Settings › Units and Zone, choose the plan's system.
4. If you'll combine it with a new WGS 84 survey, convert before mixing, never after.

If your firm gets plans like this every week, it's repetitive work worth automating. We cover it in Civil 3D automation, the complete guide.