Coordinate Reference Systems, Datums, and Projections — GIS / Surveying / Drone Operations

Objective. Students will select and apply appropriate coordinate reference systems (geographic and projected) for US surveying projects and re-project datasets correctly in both ArcGIS Pro and QGIS.

What Is a Coordinate Reference System?

A coordinate reference system (CRS) is a framework that ties a set of numbers—coordinates—to real locations on the Earth's surface. Every GIS dataset carries a CRS definition. If you open two datasets with different CRS definitions and do not account for that difference, features will not line up, distances will be wrong, and area calculations will be meaningless. For surveying and drone mapping work, a mismatch of even a few meters can invalidate a boundary determination or a volumetric calculation.

The Earth's Shape: Ellipsoids

The Earth is not a perfect sphere. Geodesists approximate its shape with a mathematical surface called an ellipsoid (also called a spheroid), defined by a semi-major axis (equatorial radius) and a semi-minor axis (polar radius), or by the semi-major axis plus a flattening ratio. Different ellipsoids have been used over history. The GRS 1980 ellipsoid underlies the datums most important to US surveyors today.

Datums

An ellipsoid is just a shape. A datum anchors that shape to the actual Earth by defining an origin and orientation, typically through a network of precisely measured ground control points.

NAD 83 (North American Datum of 1983) is the current horizontal datum for surveying and mapping in the United States and Canada. It is based on the GRS 1980 ellipsoid and is referenced to the North American tectonic plate. NGS maintains and publishes NAD 83 coordinates through the National Spatial Reference System (NSRS). State Plane Coordinate System zones and most legal survey work in the US reference NAD 83.
WGS 84 (World Geodetic System 1984) is the global datum used by GPS satellites and is defined and maintained by the US Department of Defense. For most practical field work, NAD 83 and WGS 84 coordinates are very close—often within about one meter or less—but they are not identical, and high-precision surveys must treat them as distinct. Drone GPS receivers typically record positions in WGS 84.
NAVD 88 (North American Vertical Datum of 1988) is the primary vertical (height/elevation) datum for the conterminous US. It is a separate concept from horizontal datums and is based on a network of benchmarks tied to mean sea level at a tidal gauge in Rimouski, Quebec.

Important: NGS is actively developing a modernized datum framework (NATRF2022 / NAPGD2022). Keep an eye on NGS announcements, because datum transitions affect legal descriptions and deliverable requirements.

Geographic vs. Projected Coordinate Systems

A geographic coordinate system (GCS) expresses locations as latitude and longitude in angular units (degrees). It uses the ellipsoid surface directly. Distances and areas calculated in degrees are distorted and essentially unusable for engineering or surveying.

A projected coordinate system (PCS) mathematically flattens the ellipsoid onto a flat plane using a map projection, then measures positions in linear units such as US survey feet or meters. Only a PCS lets you reliably measure distances and areas in a GIS.

Key Projected Systems for US Surveyors

State Plane Coordinate System (SPCS)

SPCS divides the US into zones, each designed to limit distortion to about 1 part in 10,000 or better within that zone. Each zone uses either a Lambert Conformal Conic or Transverse Mercator projection (with a few Oblique Mercator zones in Alaska). SPCS coordinates are in US survey feet or meters depending on the state. Legal survey plats and many government datasets are delivered in State Plane. You must select the correct zone for the project location—using an adjacent zone introduces unnecessary distortion.

Universal Transverse Mercator (UTM)

UTM divides the world into 60 north-south zones, each 6° of longitude wide. The conterminous US falls roughly within UTM Zones 10N through 19N. UTM uses meters. It is widely used for drone mapping deliverables, multi-state projects, and when working with imagery from satellite sources.

WKID Codes

Every CRS recognized by Esri is assigned a Well-Known ID (WKID), a numeric code stored in the dataset's projection file (.prj) and displayed in software. QGIS uses the same EPSG codes for most CRS definitions (EPSG is an authority whose codes Esri also supports). For example, NAD 83 geographic has EPSG/WKID 4269; WGS 84 geographic is EPSG 4326. State Plane zones each have their own WKID—always verify the correct code for your state and zone in the software's CRS search dialog before delivering a dataset.

Reprojection Workflow

Reprojection means mathematically transforming coordinates from one CRS to another. In ArcGIS Pro, use the Project geoprocessing tool (Data Management toolbox). In QGIS, use Vector → Data Management Tools → Reproject Layer. Always verify reprojection by checking that known benchmark coordinates match published NGS values within acceptable tolerance. Document the input and output WKID codes in your project metadata.

Why This Matters for Area and Distance

A dataset in geographic coordinates (degrees) cannot produce accurate area or distance measurements. A dataset in the wrong projected zone will accumulate scale error. For example, measuring a parcel area in UTM when the parcel spans the edge of a State Plane zone—without accounting for the grid scale factor—can produce errors that matter in boundary disputes or earthwork volume estimates.

Safety and Data Integrity Note

Always maintain an unmodified copy of original source data before reprojecting. Save reprojected files to a clearly named output location. Overwriting source data is a form of data loss that can compromise a survey project. Treat CRS metadata with the same care you would treat a field measurement.

Key terms10

Coordinate Reference System (CRS)A complete framework that defines how coordinates relate to locations on the Earth, combining a datum and, for projected systems, a map projection.

EllipsoidA mathematically defined solid that approximates the shape of the Earth, characterized by its semi-major axis and flattening ratio; used as the geometric foundation for a datum.

DatumA reference system that anchors an ellipsoid to the Earth through a network of control points, providing a consistent origin for coordinate measurements; NAD 83 is the primary horizontal datum for US surveying.

NAD 83North American Datum of 1983; the current official horizontal datum for the United States, based on the GRS 1980 ellipsoid and maintained by NGS.

WGS 84World Geodetic System 1984; the global datum used by GPS satellites, maintained by the US Department of Defense; very close to NAD 83 but not identical for high-precision work.

State Plane Coordinate System (SPCS)A set of US zone-based projected coordinate systems designed to minimize distortion within each zone, using either Lambert Conformal Conic or Transverse Mercator projections; commonly used for legal surveys and state/local government mapping.

Universal Transverse Mercator (UTM)A global projected coordinate system dividing the world into 60 six-degree-wide zones and using meters; widely used for drone mapping, satellite imagery, and multi-state projects.

WKID / EPSG CodeA numeric identifier assigned to a specific CRS definition by Esri (WKID) or the EPSG authority; used in software to select and record the correct projection for a dataset.

ReprojectionThe mathematical transformation of a dataset's coordinates from one CRS to another, performed in GIS software using tools such as ArcGIS Pro's Project tool or QGIS's Reproject Layer tool.

NAVD 88North American Vertical Datum of 1988; the primary vertical datum for the conterminous US, providing the reference surface for orthometric (elevation above sea level) heights.

Worked examples3

2. A rectangular parcel measured in a geographic CRS (WGS 84, decimal degrees) has corners at approximately 38.0000°N, 105.0000°W and 38.0090°N, 104.9900°W. A technician calculates the 'area' as (0.0090° × 0.0100°) = 0.000090 square degrees. Convert the side lengths to approximate meters to show why square degrees are not a valid area unit.

Step 1 – Convert the north-south dimension (latitude difference) to meters. One degree of latitude ≈ 111,000 m. So 0.0090° × 111,000 m/° ≈ 999 m (north-south side).
Step 2 – Convert the east-west dimension (longitude difference) to meters. One degree of longitude at 38°N ≈ 111,000 m × cos(38°) ≈ 111,000 × 0.7880 ≈ 87,468 m/°. So 0.0100° × 87,468 m/° ≈ 875 m (east-west side).
Step 3 – Calculate the approximate area in square meters: 999 m × 875 m ≈ 874,125 m².
Step 4 – Convert to acres for context: 874,125 m² ÷ 4,047 m²/acre ≈ 216 acres.
Step 5 – The technician's answer of 0.000090 square degrees is dimensionally meaningless and cannot be compared to acres, square feet, or square meters without the conversion factors above. Projecting the data to State Plane (US survey feet) or UTM (meters) is required for valid area output.

Answer: The parcel is approximately 874,125 m² (about 216 acres). The '0.000090 square degrees' figure is not a usable area value; area calculations must be performed in a projected CRS with linear units.

3. You receive a drone survey shapefile labeled 'WGS84 UTM Zone 13N' (EPSG 32613, meters). Your client needs deliverables in Colorado State Plane Central Zone, NAD 83 (US survey feet). After reprojecting in ArcGIS Pro, a check point that reads E = 2,483,215.34 m, N = 1,877,432.11 m in UTM should appear at approximately E = 1,342,450 ft, N = 1,728,900 ft in State Plane. Describe the verification steps you would take.

Step 1 – Before reprojecting, record the UTM coordinates of at least one known NGS benchmark from the shapefile attribute table or by clicking the point.
Step 2 – Look up the same benchmark in the NGS Data Explorer online and note its published NAD 83 State Plane Colorado Central coordinates in US survey feet.
Step 3 – Run the Project tool in ArcGIS Pro: Input CRS = EPSG 32613 (WGS 84 UTM Zone 13N); Output CRS = the correct WKID for NAD 83 State Plane Colorado Central (US survey feet). Verify the correct WKID in ArcGIS Pro's coordinate system search before running.
Step 4 – After reprojection, click the same benchmark point in the output layer and read its new coordinates from the attribute table or the Identify tool.
Step 5 – Subtract the GIS-reported coordinates from the NGS-published coordinates to find the residual error. For standard GIS work, a residual under 0.1 ft (about 0.03 m) is acceptable; for legal surveys, follow the applicable state standard.
Step 6 – Document the input WKID, output WKID, benchmark ID, published coordinates, measured coordinates, and residual in the lab worksheet.

Answer: Verification consists of comparing reprojected GIS coordinates for a known NGS benchmark against published NAD 83 State Plane values, computing the residual error, and confirming it falls within acceptable tolerance. Record all WKIDs and residuals in the project metadata.

The math in this lesson2

In class: Lecture and diagram exercise: students sketch the relationships among ellipsoid, datum (NAD83, WGS84), and projected coordinate systems (State Plane, UTM) on a blank diagram and label key parameters. · Lab exercise: students open a shapefile projected in WGS84 Geographic, reproject it to the correct State Plane zone for their state in ArcGIS Pro, verify coordinate values against a known benchmark, and record the WKID codes used. · QGIS parallel exercise: students repeat the reprojection workflow in QGIS using the 'Reproject Layer' tool, then overlay both outputs to confirm spatial alignment and document any offset.
Materials: ArcGIS Pro, QGIS, sample shapefiles (county boundary, control points), NOAA/NGS Datums fact sheet, State Plane zone reference table, lab worksheet
Check: Students submit a lab report showing correct WKID selections, before/after coordinate screenshots, and a written explanation of why projection matters for area and distance measurements; pass mark 75% on a graded rubric.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.