# What units are my coordinates in?

I opened a georeferenced NAIP photo to a new project in QGIS. The coordinates were expressed as: 2199240.6, 616858.7. The same point in Google Earth was expressed in degrees, minutes, seconds, as 41°51'25.33"N, 76°54'12.14"W. In what form is QGIS displaying the coordinates? Are these correct?

When I opened the NAIP tif in QGIS, the CRS set itself to NAD83/Pennsylvania North.

• Does the NAD83, Pennsylvania North projection make sense? Seems like it should: spatialreference.org/ref/epsg/2271 This says feet. – Alex Leith Aug 21 '13 at 4:35
• there is also a PA North wich is in meters (for modern americans) - see spatialreference.org/ref/epsg/3362 – Ian Turton Aug 21 '13 at 8:27
• @iant: true, but the given coordinates do not fit into the projected bounds, while they do in the "feet" version. I guess thats intentional to avoid confusing with both systems. – AndreJ Aug 21 '13 at 9:05
• Wow - I'd never noticed that (though I never really got the USFt thing) – Ian Turton Aug 21 '13 at 9:15

## 4 Answers

EPSG:2271 looks good. Just transform your deg-min-sec coordinates into decimal degrees, put them in a text file like this:

Nr,Y,X
1,41.857,-76.903
2,42,-77


Add it to the QGIS project with layer CRS EPSG:4326. Then change the project CRS to EPSG:2271, and read the coordinates at the first point. Matches perfectly to the coordinates you wanted.

So the photo has US feet as map units.

The coordinates in the status bar are always those of the project CRS, which does not have to be the same as the layer CRS (and every layer may have a different one).

The units for your data are meters. A quick search for meta data on NAIP imagery yields this link which confirms the CRS as NAD83 and the planar distance units as meters, which is corrorborated by this information sheet.

To avoid confusion, the world file of your Tiff won't tell you what the units are. It will merely tell you how many units there are per pixel (whether they be feet or meters etc). In your case the nominal ground resolution according to the metadata is 1m and this is confirmed by what you report is in the world file. You can get more information on world files here.

Depending on what type of image file you have. Geo tiff the position is in the file (header) tif there is a tfw. That determines the position rotation and scale when inserted.
It can be opened in notepad. It will NOT tell you the crs. You will need to determine the epsg code and see all the details in the epsg database.

Qgis has a setting to enable on-the-fly projection which means it allows multiple data layers in multiple projections in the same document. (Which is done by most all GIS systems.

There are some apps on arcgis.com that help you determine coordinates and projections.

• I opened the .tfw in notepad. It gave me the following readout: 1.000000 0.000000 0.000000 -1.000000 2190000.500000 619999.500000 ... The name of the folder the files came in was 62002190PAN.zip. Clearly, 2190000.5 and 619999.5 are important numbers. When I move my cursor over the image, the ranges of the coordinates are close to those numbers. I am new to GIS and I am still confused. Are those numbers in a form of longitude/latitude lines? They are supposed to be above northern PA, but they arent. How can I make sense of those numbers? – Ryan Aug 21 '13 at 5:31
• The worldfile (.tfw) specifies the position of your images. The x-coordinate of the center of the upper left pixel is 2190000.500000 (and y is 619999.500000). This position is specified for a specific coordinatesystem. But the wold file does NOT tell you which coordinatesystem. The coordinatesystem is specified in a .prj file (you can open it in a text editor). But often there are no .prj files. – Jens Aug 21 '13 at 6:07
• To be honest, I have never seen a raster file with a .prj file. prj are only common for shapefiles. – AndreJ Aug 23 '13 at 14:10

It's either this: http://spatialreference.org/ref/epsg/2271/ or this: http://spatialreference.org/ref/epsg/3362/ so either feet or metres, respectively.

EDIT: sorry, I missed the second link. There are two Pen North reference systems, one in feet, one in metres.

• What would the coordinates in feet be in reference to? is there a way I could convert this to degrees or a form my garmin etrex GPS uses? – Ryan Aug 21 '13 at 6:13
• @Ryan: The GDAL tool cs2cs can convert between any coordinate systems. – AndreJ Aug 21 '13 at 9:07
• @Ryan, if you have your file in QGIS, you can right click it in the layers list and choose 'save as' then choose WGS84 (EPSG:4326) aas the CRS. That will work in your Garmin. (I don't know what format your device needs, though.) – Alex Leith Aug 21 '13 at 23:42