Map Projections Explained: Transverse Mercator, Lambert Conformal Conic and Albers Equal Area

No projection keeps shape, area, distance and direction at the same time. Conformal projections keep angles, so small shapes look right and a bearing read off the map is the bearing on the ground — that is what survey work needs. Equal-area projections keep area, so a hectare is a hectare anywhere on the map, at the cost of shape. Scale changes across every projection; the scale factor says by how much at a given point.

A projected coordinate reference system combines a projection with a datum and a unit, and its EPSG code names the whole combination. That is why two datasets can both say "UTM zone 35N" and still disagree: their datums differ.

Transverse Mercator wraps a cylinder around the earth so that it touches along a central meridian. Scale is almost true in a narrow north–south band either side of that meridian and grows with distance east or west. The projection is conformal, which is why most survey grids are built on it.

UTM (Universal Transverse Mercator) divides the world into 60 zones, each 6° of longitude wide, and multiplies the scale on the central meridian by 0.9996 to spread the error across the zone: grid distances are about 0.04% short on the central meridian, true roughly 180 km either side of it, and up to about 0.1% long at the zone edge near the equator. National grids use the same projection with other zone widths and scale factors — Turkey, for example, uses 3° zones with a scale factor of 1 on the central meridian.

Lambert Conformal Conic sets a cone over the earth, usually so that it cuts the globe along two standard parallels. Scale is true on those parallels, slightly small between them and slightly large outside, and it changes with latitude only. That makes the projection a good fit for mid-latitude regions that are wider east–west than north–south, where a single Transverse Mercator zone would be too narrow. Like Transverse Mercator, it is conformal.

Many national and state grids are built on it — France's Lambert-93 (EPSG:2154) is one — and so are many aeronautical charts.

Albers is also a conic projection with two standard parallels, but it keeps area instead of angles. Every unit of area on the map stands for the same area on the ground, which makes it the projection for statistics and thematic maps over mid-latitude extents: land cover, catchment areas or forest stands compared across a region. The contiguous United States (EPSG:5070) and Australia (EPSG:3577) both have standard Albers systems.

Because shapes and distances bend to keep the areas right, Albers is not a projection for setting out or for measuring lengths. Data delivered in it is usually reprojected to a conformal grid before engineering work starts.

Measure in a conformal projection close to the site: the local Transverse Mercator zone, the national grid, or a low-distortion projection defined for the project. Keep the same projected CRS for every dataset in a project — DEM, orthophoto, point cloud and CAD — so the numbers you compare are in the same grid.

The projection works on the ellipsoid, so a grid distance differs from a ground distance by the scale factor and by the height above the ellipsoid. Where a contract asks for ground distances, apply the combined scale factor rather than assuming the grid is the ground.

STREAM reads the coordinate system that a GeoTIFF, a LAS/LAZ file or a project declares and resolves it to an EPSG code, so a UTM or national Transverse Mercator zone, a Lambert Conformal Conic grid and an Albers system are all read the same way. Distances, areas and volumes are measured in the grid units of that CRS.

Keep one projected CRS per project. When a raster was delivered in a different projection from the rest of the data, reproject it before loading so that every layer shares the same grid.