An orthophoto is an aerial image from which the distortion caused by terrain and camera tilt has been removed, so you can measure distances and areas on it the way you would on a map. An orthomosaic is the same result stitched together from hundreds of photos into one continuous sheet. GeoTIFF is the file format it is usually delivered in: an ordinary image with georeferencing built in.
Below: how an orthophoto differs from a regular photo and from an orthomosaic, what is inside a GeoTIFF, what drives quality, which coordinate systems are used in Georgia, and how to check the file before you accept it.
How an orthophoto differs from a regular aerial photo
On a regular photo, tall objects and slopes appear shifted: the taller the object and the farther it is from the center of the frame, the more it “leans”. The scale also varies across the frame, so you cannot measure on it with a ruler. Orthorectification recalculates every pixel using the height of the surface and turns the image into a true top-down view.
- Regular photo. Easy to read but distorted and not tied to coordinates. Good as an illustration.
- Orthophoto. A real image of the site at a uniform scale and tied to coordinates. Suitable for measurement and as a base layer in CAD and GIS.
- Vector map or plan. Symbols and lines without photographic detail. An orthophoto is often the base from which they are drawn.
Orthophoto vs orthomosaic: are they the same thing?
In professional use the difference is this. An orthophoto is a single orthorectified image. An orthomosaic is the result of stitching many such images into one continuous sheet.
A drone survey always produces a mosaic, so in orders the two words are almost interchangeable. It matters more to specify the parameters of the result: pixel size, coordinate system and verifiable accuracy.
How an orthophoto is made
- Flight with overlap. Photos typically overlap by about 70–80% along the flight line and 60–70% between lines, so every ground point appears in several images.
- Georeferencing. RTK corrections give the coordinates of each photo, while ground control points (GCPs) and checkpoints on the site fix and verify the result.
- Surface model. A point cloud and a digital surface model (DSM) are built from the photos. This model is what lets the image be “straightened”.
- Orthorectification. Every photo is recalculated using the surface model.
- Mosaicking and color balancing. Photos are assembled into one sheet, seams are blended and brightness is evened out.
- Export. Pixel size, coordinate system and file format are set.
For more on the survey itself, see Drone Surveying in Georgia: What It Is, How It Works and What You Get.
What is a GeoTIFF?
A GeoTIFF is an ordinary TIFF file with extra information stored inside: the coordinate system and the position and size of the pixels. That is why QGIS, ArcGIS, Global Mapper and other software place the image in the right spot by themselves, with no manual alignment. Sometimes a separate world file (.tfw) sits next to the image: same purpose, but the data is stored separately.
Do not mix them up: a JPEG or PNG exported from an orthophoto is just a picture. Without georeferencing you cannot measure anything in coordinates on it.
Terrain and surface models are delivered in the same format. The difference is in the content: in an orthophoto a pixel stores a color, in a terrain model it stores a height.
- File size. An orthophoto of a large site can run to several gigabytes. A standard TIFF is limited to roughly 4 GB, so BigTIFF is used for larger files.
- Fast viewing. A file with overviews (reduced copies) or in Cloud Optimized GeoTIFF (COG) format opens noticeably faster.
What determines orthophoto quality
- Ground sample distance (GSD). The size of one pixel on the ground. The smaller it is, the more detailed the image and the heavier the file. A few centimeters per pixel is often enough for large sites; detailed plans call for less.
- Georeferencing accuracy. It is not the same as pixel size. A 2 cm pixel does not mean a 2 cm error in the plan. Real accuracy is shown by the deviation from independent checkpoints (RMSE).
- Coordinate and height system. What is used in Georgia is covered in the next section.
- Seams and color. Visible joins, “steps” on straight lines and brightness shifts point to weak processing.
- Type of orthorectification. On a standard orthophoto the edges of tall buildings lean slightly. A “true ortho” removes this effect, but facades are then not visible.
Coordinate systems in Georgia
In Georgia, data is commonly delivered in WGS 84 / UTM zone 37N or 38N. The boundary runs near 42° E: the west of the country, including Batumi, falls in zone 37N and the east, including Tbilisi, in zone 38N. Cadastral and government tasks may require the national Georgia Geodetic Datum (GGD). That is why the system is agreed before the flight with you and, if needed, with the receiving authority.
What an orthophoto does not show
- Heights. It is a flat image. For heights you need a digital terrain or surface model.
- Ground under tree canopy and roofs. The camera sees only what is visible from above.
- Moving objects. Cars and people can leave “ghosts” where images are joined.
- Water, snow and uniform surfaces. These areas reconstruct poorly from photos and may show artifacts.
What orthophotos are used for
- Design. A base layer in CAD showing real site conditions.
- Construction. Comparing site status between dates and checking progress by stage.
- Cadastre and land management. A base for refining boundaries. Whether it is acceptable for registration depends on the requirements of the specific authority, so these are clarified before the flight.
- GIS and municipal management. Updating layers, inventory of parcels and infrastructure.
- Agriculture and winemaking. Irrigation and drainage planning, monitoring the condition of plantings.
How to check an orthophoto before accepting it
- Open the file in your own software or in free QGIS. It should land in the right place in the right coordinate system.
- Compare a few known points. Building corners, manholes, poles: the deviation should fall within your tolerance.
- Zoom to 100%. Look for blur, seams and “steps” on straight lines.
- Ask for a checkpoint report with horizontal deviations.
- Check the size and format. The file should open in your software without freezing.
We survey a test area of up to 0.5 ha for free, so you can check the orthophoto in your own software before the project starts.
What to specify in your order
- the purpose: design, cadastre, monitoring, GIS;
- the pixel size or plan scale you need;
- the coordinate and height system;
- the format: GeoTIFF, Cloud Optimized GeoTIFF or another, and whether to split into tiles;
- related deliverables: terrain model, point cloud, topographic plan.
More about the service: orthophoto mapping in Georgia.
Orthophoto and GeoTIFF: frequently asked questions
How is an orthophoto different from a satellite image in online maps?
A satellite image in online maps gives a general picture of the area, but its date may be out of date and its positional accuracy is not guaranteed for measurement. A drone orthophoto is captured for your task, has verifiable accuracy and higher resolution.
How accurate is an orthophoto?
There is no single figure. Accuracy depends on the pixel size on the ground, the quality of georeferencing and checkpoints, the terrain and the processing. Pixel size is not the same as error: with a 2 cm pixel the horizontal deviation can be larger. Ask for a report of deviations against independent checkpoints.
Which coordinate system is used for orthophotos in Georgia?
Usually WGS 84 / UTM, zone 37N for the west of the country including Batumi, or zone 38N for the east including Tbilisi. Cadastral and government tasks may require the national Georgia Geodetic Datum (GGD). We agree the system before the survey.
Which programs open a GeoTIFF?
Most GIS and CAD programs, for example QGIS, ArcGIS, Global Mapper and Civil 3D. They read the georeferencing stored in the file and place the image correctly. In ordinary photo viewers the file opens as a plain picture, without coordinates.
Why is an orthophoto file so large, and can it be made smaller?
Size grows with area and detail: the smaller the pixel, the heavier the file. It can be reduced by choosing a larger pixel size, applying compression, splitting the site into tiles or delivering a format with overviews for quick viewing. It is best to agree the parameters before the flight.
How is an orthophoto different from a 3D model?
An orthophoto is a flat top-down image on which you can measure distances and areas. It does not store heights. A 3D model and a point cloud convey volume and shape, while terrain heights are delivered as a separate file: a digital terrain or surface model.