Below are some frequently asked questions by clients of the photogrammetric industry. If you can't find the answer to your question, please click here to submit a question and a Surdex representative will send you a reply or you may call 636-532-3427.

1. What is the difference between large-scale mapping and small-scale mapping?

In terms of map scale, you might expect a 1"=50' or 1"=100' map to be considered "smaller" in scale than a 1"=1,667' or 1"=10,000' map. However, that is not the case. Large-scale maps are generally used for applications that require detailed (not generalized) information. Small-scale maps are, conversely, generally apply well to regional or large area usage. Large-scale maps are generally considered to be all scales larger than 1:20,000 or 1"=1,667'.

2. What are the most common map scales and what applications are they most associated with?

The products that are needed by your community as determined in your needs assessment will dictate the map scale to be produced. Many features like street centerlines, edge of pavement and buildings can be captured from many different map scales ranging from 1"=50' to 1"=400'. Typical map scales for municipal mapping applications are 1"=100' for urban or developed areas and 1"=200' or 1"=400' for rural and less developed areas. However, should it be determined that features like fire hydrants or manholes are needed by the users, your community should consider an alternative such as 1"=50' or larger. It is important to select a mapping scale that ensures you can identify each feature you want to collect. Equally important is not to procure a scale that costs more without any practical benefit. If your community is unsure what types of features can be captured from each output mapping scale, please consult a photogrammetrist.

3. For digital orthophotography, what pixel resolutions generally correlate to various map scales?

Although these pixel resolutions are typical, your needs may dictate a higher or lower level of output pixel resolution.

4. There are so many accuracy specifications in this industry. Which are most common and how do I choose the accuracy specification that best suits my project?

Accuracy standards vary in complexity and usability. The most commonly used data accuracy standards for county and municipal mapping applications are the American Society of Photogrammetry and Remote Sensing (ASPRS) Class I and II. Additionally, more and more counties and municipalities are requesting their mapping projects to be compliant with the National Map Accuracy Standards (NMAS) for large-scale mapping.

The American Society of Photogrammetry and Remote Sensing (ASPRS)developed a new set of accuracy evaluation criteria in the early 1990s. These accuracy standards for large-scale maps (generally 1"=1000' and larger {i.e. 1"=200', 1"=100', etc.}) look at continuous datasets (not map sheet based)from a statistical perspective (the root mean square error or RMSE) and therefore are considered more stringent.

The National Map Accuracy Standards (NMAS) were created in 1947 by the US Office of Management and Budget (OMB) and oriented towards review of data on a map sheet basis. By definition, on a given map sheet, 90% of planimetric features should fall within 1/30 of the state scale - this means that on a 1"=100' scale map, planimetric features need to be no less accurate that an absolute position of 3.33'. For topographic features, data should be accurate to half the stated contour interval - meaning that on a map showing 2.0' contour interval data, all contours need to be within 1.0'. In terms of RMSE (like the ASPRS standards), NMAS generally equates to ASPRS Class 1.5. Members and clients of the photogrammetric community often misunderstand the United States National Map Accuracy Standards. Often clients ask for products to meet NMAS without clearly stating their interpretation of NMAS. Surdex recommends that each client carefully examine each potential vendors interpretation of the NMAS. By not doing so, he or she could potentially receive a mapping product that does not meet the product accuracy expected.

More recently the Federal Geographic Data Committee (FGDC) has been working towards establishing a national set of standards to replace/update NMAS to reflect today's technology and facilitate data sharing among all levels of government. Visit http://www.fgdc.gov/standards/standards.html to assess the latest developments of this initiative.

To learn more about accuracies and how they relate to photogrammetric mapping, please refer to the US Army Corps of Engineers Manual for Photogrammetric Mapping (EM-1110-1-1000).

5. I believe I need contours. What are the general uses for different contour intervals and which one is right for my project?

The following table is excerpted from the USACE Engineering and Design Manual (EM-1110-1-1000) for Photogrammetric Production. It provides the recommended use for contours. Surdex recommends that your community closely evaluate the purposes of each contour interval as listed below and choose the contour interval that is appropriate for your mapping needs as determined during the needs assessment.

It should be noted that the contour interval selected will significantly affect the cost and schedule of the project. It, along with product accuracy, will greatly influence photo scale, which will affect the cost and scheduling of aerial photography acquisition, stereo compilation, and quality control.

6. Will the direction of the aerial flight have any adverse affect on my project?

All digital orthophotography, like most other digital mapping products, is produced with all pixels having a "North-Up" orientation. With a North-to-South or South-to-North approach, the image pixels are more closely aligned to the final ortho pixel grid. The result is that a vendor can scan the North-to-South imagery in less time and at a courser resolution than would be required to create a "North-up" pixel orientation from East-to-West or West-to-East imagery.

Some may argue that scanning East-to-West imagery at a finer resolution will achieve the desired results, that it will produce a more desirable product; this, however, is not the case. There is a great advantage to scanning the original imagery at a resolution as close to the output product resolution as possible. Each time you are forced to resample the imagery (whether it be from a finer resolution to a courser resolution or vice versa), the imagery will lose quality and accuracy. Using North-to-South imagery, a vendor can scan at a rate much closer to the desired output ortho resolution. Also, by scanning at a courser resolution, vendors can scan and dodge the product in a more timely and economical manner. By scanning at a finer resolution as would be required with the East-to-West imagery, it is possible to increase the amount of data to be handled during the production process significantly. This would greatly affect the time and cost for each step of the production process including Aerial Triangulation, Stereo Compilation and Digital Orthorectification.

Not only is a North-to-South or South-to-North approach more economical, it also yields a better quality product. When flying a project from East-to-West or West-to-East, you encounter a phenomenon known as Bi-Directional Illumination. This is the relationship between the Sun Angle and the Viewing Point. Since the Sun moves from East to West, alternate flight lines will have different directionality on the shadows, which could affect the overall quality of the orthophotography. A North-to-South approach will yield a much more even illumination than the East-to-West product would produce.

7. If my city/township/county was mapped in 1997, should I update the data or redo the mapping project altogether?

Lots of things must be considered when making a decision to remap or update your data. Just a few things to consider are: how much growth/change has occurred since the original data was mapped?, was the data tied to a stringent geodetic network?, how did your contract approach the project (i.e. technology used, data collection/delivery rules, etc.)?, what is your level of confidence with the current data?, what accuracy standards were employed?, and what do you want to do with your data?. Answers to the previous questions and a whole host of others need to be addressed before making a decision one way or another.

8. Can an IMU (Inertial Measurement Unit) replace the need for ground control and/or aerial triangulation?

The IMU is a wonderful tool which provides a tremendous amount of additional information about the aircraft when the photography was taken; however, our experience does not indicate that ground control and aerial triangulation should be abandoned. For one thing, ground control is an inexpensive insurance policy for your project - except in extreme cases when placing conventional ground control targets or when photo identifiable points are not possible. Although the POS data provides orientation information about the aircraft when the exposures were made, the only way to verify that the POS data will meet certain accuracy objectives is through aerial triangulation and/or field checking with survey equipment. So why have POS data? Again, the data become a great quality control element for each project.

9. Why does vector data not always line up perfectly with the corresponding digital ortho?

Several factors impact why data may not line up perfectly with one another. One basic factor is radial displacement - the farther away from lens center, the more displacement occurs. Another reason can be attributed to the process by which the data was collected; for instance, if an organization wanted just to have digital orthos of their project area created and no mapping done, a contractor would generate a digital elevation model (DEM) specifically for the orthorectification activities. Assuming that the project was well planned, the result would be an ortho conforming to a set of standards. Then, lets say that sometime later, the same organization wanted planimetric features collected and contours generated from the same photography. So again, the contractor collects the breaklines and mass points for the contours and all the requested planimetry and delivers the data to the client meeting a certain accuracy standard. So here we have two separate data products, both conforming to a respective set of standards, but not necessarily aligning with one another perfectly. This dilemma could be resolved if the client requested the contractor to go back and re-orthorectify each image with the more dense digital terrain model (DTM) data - this would add time and cost to the project, but because of the process by which the data was requested, the problem could not have been
avoided.

10. What impacts the quality of a digital ortho image?

Many factors impact the quality of a digital ortho image including, but not limited to the film being used, the camera & lens assembly (including a gyro-stabilized mount and Forward Motion Compensation), what conditions existed when the film was exposed (day, time, atmospheric moisture, etc.), how the film was processed, quality/accuracy of the photogrammetric scanner, dodging of the film, the orthorectification software, cut line placement, histogram matching, global balancing, and finally image compression.

11. What is the difference in a surface model created with traditional photogrammetric techniques versus LiDAR?

When creating a surface model using traditional photogrammetric techniques, a series of mass points and breaklines are collected depending on the intended use of the surface model - use dictates density and characteristics of the surface model (i.e. DTM for contours and orthos, DEM for orthos). Because a human is doing the feature collection, only surface points and lines are collected. LiDAR, on the other hand, uses a laser to collect a tremendously dense series of points (mass points) that describe the Earth's surface as well as vegetation and human-made features (i.e. buildings, bridges), which must be removed from the dataset if a bare-earth surface model is the desired end product. Missing from the LiDAR surface model are breaklines, which help further describe and connect major features in the project area. This isn't to say that a LiDAR surface model can't be supplemented with breaklines.