|This is the second article in a series about writing good Request for Proposals (RFPs) to ensure high quality geospatial services. In the previous issue, we discussed some of the advantages and disadvantaged of RFPs and how to avoid some of the typical mistakes made by requesting incomplete information. In this edition, we will address geospatial specifications.|
No other item effects geospatial RFPs more than positional accuracy specifications. Buyers often leave out or inadequately define these important measures. When not clearly specified, it may be difficult or impossible to compare approaches and costs between contractors. Such lack of specificity results in sub-standard services.
What Are Positional Accuracy Standards?
Positional accuracy is typically the single most expensive component of your geospatial project. During the initial project planning stages, it is important to understand how users will interact with and use the data. This, in turn, will dictate what horizontal and vertical positional accuracies are needed and what can be afforded. Since the cost of positional accuracy increases with accuracy, understanding the needs versus wants of the user community is important to avoid excessive cost (and accuracy) and maximize value.Guidance from a professional geospatial provider is important at this stage both to grasp the relevant user issues that have a bearing on needed positional accuracy and to determine how much accuracy is affordable. Misjudgment in how much (or how little) accuracy is required and poor project design are the most common reasons geospatial projects fail. A contractor selection process maximizing relevant professional expertise will also maximize the value of the procurement for you and your user community. It is important the RFP state a certified photogrammetrist on staff is a requirement for all bidders.
These professionals receive national certification from the American Society for Photogrammetry and Remote Sensing (ASPRS) every three years per educational/experience requirements. This professional credential maximizes the odds the professional has a thorough understanding of positional accuracy specifications and mapping project design.
Understand the Positional Accuracy Standards
Understanding the different positional accuracy specifications will help you discuss your project. The three most common specifications include:
- National Map Accuracy Specifications (NMAS)
- American Society of Photogrammetric and Remote Sensing Specifications 1990 (ASPRS)
- National Standard for Spatial Data Accuracy (NSSDA)
|Comparison of Horizontal Accuracy Standards: NMAS & ASPRS1|
|Horizontal RMSE (Feet)|
|ASPRS Class 1||NMAS Class 1.5||ASPRS Class 2|
|The maps scales shown here are commonly used for large scale mapping projects. So these figures indicate, for example, that the 1″=100′ scale mapping compliant with National Map Accuracy Standards would have a horizontal accuracy greater than or equal to +/- 1.5 feet.|
Specifying Positional Accuracy
The specific accuracy standard specified in your RFP is less important than defining the spatial accuracy for each given deliverable. Consider required spatial accuracy carefully based on the anticipated needs of user community. Spatial accuracy is expensive, so identify accuracy needs not wants. There normally will be a tension between accuracy and budget. By balancing spatial accuracy needs with budget realities, you can provide the best value. Your professional geospatial service provider can also help you identify the best value using current geospatial technology.As mentioned, it is important RFPs specify the required spatial accuracy for deliverables. If not, the photogrammetric consultants will not have a common baseline for making the important decisions about scale, precision, detail, and cost. The result for the buyer will be proposals with such widely varying positional accuracy and costs that it will be difficult or impossible to realistically compare the value of each proposed project approach.
Then, Leave “How To” To Provider
The RFP ideally should only specify the desired spatial accuracies. For instance, the photo scale, flight height, and equipment should generally not be dictated by you. Leaving this control with the geospatial professionals allow them to maximize your value. This has become increasingly important as digital sensors and airborne GPS technology have come to market. These technologies enable achieving specific positional accuracies using vastly different project parameters than could be done historically with analog (film) equipment.Set your required spatial accuracies, insist on receiving references, and request previous project experience, but leave the “how to achieve your spatial accuracy” to the vendor.
Government Cooperation, More Value
Public agencies considering procurement of digital mapping services should consider cost sharing with other government sources. Federal agencies often need quality data and will share acquisition or conversion cost if the data can be standardized to benefit all users. In fact, because the Federal government insists on standardized data, the mapping data is often more useful to more user groups than envisioned.An example of such a program is FEMA’s digital flood insurance rate maps (DFIRM). The agency has established a cost-sharing program through its Cooperating Technical Partners (CTP) for local governments (www.fema.gov/plan/prevent/fhm/ctp_main.shtm). The National States Geographic Information Council (NSGIC) is also promoting the Imagery for the Nation program to facilitate data/cost sharing for geospatial services. The USDA APFO has been sponsoring Federal/State cooperative projects for several years through its National Aerial Imagery Program (NAIP).
Positional Specifications Lead To Best Value
By understanding positional accuracy specifications and including them in your next request for proposal, you can be sure your project data is accurate and represents the best value possible. Then, understand the user communities’ needs and balance them with budget realities. If you are in doubt, ask for help from a trusted certified photogrammetrist.
Look For Part 3 In The Next Issue
In the Part 3 of this RFP series, we will discuss how to indicate proper use of ground control. Learn how these processes work and what aspects are important to mention in Part 3 of our RFP series.
Questions Or Comments?
If you have questions about this article, feel free to contact Aerial Services, Inc.’s experts for advice with no obligation. We are here to help you.References
- USACE Engineering and Design Manual for Photogrammetric Production