Measuring ground features in imagery, called "image mensuration", is an important function in many image interpretation or feature compilation applications. Image mensuration is defined as applying geometric rules to determine distance, area of a 2-dimensional or 3-dimensional surfaces using the information obtained from lines and angles. It also includes measuring the height and absolute location of a feature.
Imagery from modern sensors contain important metadata determining the location and attitude (roll, pitch and yaw) of the sensor platform when the image is collected. Using this information, georeferenced imagery can be used to provide measurements. Measurements can also be collected using imagery displayed in image space rather than map space. These include basic measurements such as distance or area. Additionally, height measurements can be obtained when a sensor model exists, and measurements using shadows visible in the image can be determined using sun angle information. All measurements may be refined for more accurate results by using DEM data to account for terrain effects.
Making measurements using imagery
ArcGIS Pro provides a set of tools for image mensuration, including tools to measure distance, point location, angle, height, perimeter, area and centroid of area from georeferenced raster and mosaic datasets with sensor information.
|Tool||Tool Name||Spatial reference system||Camera model||Sun information|
Base to Top Height
Base to Top Shadow Height
Top to Top Shadow Height
Measurements can be recorded in the Measurement Results pane, and are saved with the project.
Once your Imagery is georeferenced, you can perform various basic measurements:
- Point tool— determines the location of a point based geographic coordinates.
- Distance tool—measures the distance between two or more points. When there are multiple vertices within a line, the cumulative distance of the polyline is measured.
- Area tool—calculates the size and perimeter of an interactively drawn polygon defining an area of interest.
- Centroid tool—determines the centroid of the area of interest defined by an interactively drawn polygon.
- 3D Point tool—measures the location of a point along with its elevation.
- 3D Distance tool—measure the distance between two or more points while incorporating an elevation surface.
- 3D Area tool—calculates the size and perimeter of an area while incorporating an elevation surface.
- 3D Centroid tool—calculate the centroid location of an area while incorporating an elevation surface.
When an elevation dataset is defined in the Mensuration Options pane, it will use a DEM to refine the calculations for all the measurement types.
You can measure the height of objects in the image if the sensor model of the image is available. Additionally, if sun angle information is available in the metadata, you can make measurements using the shadows visible in the imagery. See the table summarizing which mensuration modes are supported by which raster types and products.
- Base To Top Height tool—Calculates the height of a ground feature by measuring from the base of the object to the top of the object. Measurements are assumed to be perpendicular to the base; therefore, the line being measured along the building must have its end point directly above the start point. Measurement will not be as accurate for object that taper or lean from the base.
- Base to Top Shadow Height tool—Calculates the height of a feature by measuring from the base of the object to the top of the object's shadow on the ground. The point in the shadow must represent a point on the visible object that is perpendicular to the base.
- Top to Top Shadow Height tool—Calculates the height of a feature by measuring from the top of the object to the top of the objects's shadow on the ground. The measurement points on the object and its shadow must represent the same point. This tool is useful for obtaining the height of an object on the top of a structure, for example, if there was a smaller room than the dimensions of the building or a tower on the top. You can identify the top point of this feature in the image and identify the same point in the shadow.
Increase Measurements Accuracy
Specify an elevation dataset in the Mensuration Options pane to calculate more accurate distance and height measurements. Otherwise, the distance or height measurements provided is projected distance or height, measured only using the information in the image.
Using an elevation dataset modifies the distance measurement to provide a measurement that accounts for terrain which may have a significant influence on the measurement; otherwise, the distance provided is a projected distance, measured only using the information in the image.
More about image mensuration
The mensuration tools are exposed on the main Imagery tab in the Mensuration group. Expand the group to display all the available mensuration tools. The type of measurement tools available depends on the information in the metadata associated with the imagery. Basic measurements can be performed on georeferenced imagery. Measuring the height of a feature requires a sensor model, and measuring feature height based on shadows visible in the imagery requires sun angle information in the associated metadata.
The table below describes the mensuration capabilities of a raster product or the data added to a mosaic dataset using a raster type. Raster products or raster types have more capabilities since they can have information stored in their metadata that allows the application to derive the height information from them
|Raster type or raster product|
Landsat 5 TM
Landsat 7 ETM+
SPOT Scene 1A
SPOT Scene 2A
SPOT Scene 1A
SPOT Scene 2A
On the Imagery tab, in the Mensuration group, click to open the Mensuration Options dialog.
Units for the different Measurement Types can be selected in the Mensuration Options dialog. Additionally, symbols and colors for the different measurement types can be selected, and the elevation file for more accurate and 3D measurements can be identified.
Measurement accuracy can be increased by assigning an Elevation dataset from the file navigation icon. Either an elevation file or the URL address of an elevation service can be specified.
The mensuration tools provide interactive methods to measure the heights of objects in imagery. The Constrain direction of cursor movement for height measurement tools option in the Mensuration Options dialog constrain the cursor movements to make it easier to make height measurements.
The three height measurement tools operate such that the initial measurement point becomes the endpoint of a line for the constraint. The cursor will only move along the line that is appropriate for the measurement. For instance, when the Base To Top Height measurement tools is chosen, the first point should be the base of the object. The cursor movement is then constrained in the vertical direction. This is important when measuring objects where the location of the base is difficult to discern from the top of the object. For example, it may be easy to find the edge of the base of a pyramid type of feature, but difficult to determine the center of the base. There are also instances where other endpoints of the measurement are difficult to see and the constraint can assist the user in identifying them.
For the Base To Top of Shadow Height tool, the initial point should be the base of the feature. The cursor will then be constrained to move along the path of the shadow on the ground.
For the Top to Top Shadow Height tool, the initial point should be the top of the feature. When active, it will constrain the cursor to follow the path of the sun rays that form the top edge of the shadow from the top of the feature.
The measurement constraints are a convenient way to use the height measurement tools, and help you to collect correct measurements using the image's metadata.
Saving your measurements
The different measurement types are saved in the project's geodatabase as the appropriate feature classes. For example, point and centroid measurements are saved as a point feature class, distance and height measurements are saved as a polyline feature class, and area measurements are saved as a polygon feature class. The location and appropriate feature class type is handled within the project file so there is no need for the user to locate or manage these files.
Measurements are saved when a project is explicitly saved, or upon exiting the project whether the project is saved or not. When the project file is loaded in ArcGIS Pro, the automatically saved mensuration results are also loaded and available for viewing or editing in the mensuration Results pane. The measurements are linked via the feature class's attribute table to the appropriate source image file.
Changing the default location of the project's geodatabase file may cause previous measurements to not appear up in the results pane.
Measurement Results pane
Mensuration results can be viewed using the Measurement Results pane. You can access the pane from the Imagery tab.
The Measurement Results pane records information about all the mensuration results for the selected map such as mensuration Type, Name, and Description. When you select a particular measurement, the result for that measurement is highlighted. Additional metadata about the measurement such as Sensor Name, Sensor Model, and Time of measurement is also displayed.
You can use the tools on top of the measurement table to zoom to a measurement, delete the selected measurement, open the mensuration Options page as well as filter measurements based on type.
A report listing your measurements can be generated and saved. Click on the Report icon on the Measurement Results pane to name and save your report. All selected measurements in the Measurement Results pane will be written to a text file. If no results are selected, all measurements for the map will be written to the report.
The information recorded for each measurement includes:
- Image Source—The name and path to the image being analyzed
- Measurement result
- Sensor model
- Time of measurement—the time when the measurement was collected
- Type of measurement
- Coordinates—measurement in map coordinates (xyz)