Polk County Auditor

GIS / Mapping

The GIS / Mapping division of the Polk County Auditor is responsible for the maintenance of the parcel basemap that is used by various agencies throughout Polk County.

What accuracy should I expect?

The accuracy of the parcel basemap is tied to our original project and the accuracy of the digital orthophotos base.  Currently the Polk County Assessor receives digital orthophotos every other year at a published scale of 1”=100’.  The National Map Accuracy Standard for this scale is 90% of the points will be within ± 3.3 feet or 1/30 of an inch at published scale.  Our parcel maps are constructed upon this orthophotos base and therefore we cannot exceed the accuracy of our initial base. 

Formula used to test absolute map accuracy

Sqrt [(X coordinate of map – X coordinate of control) 2 + (Y coordinate of map – Y coordinate of control) 2 ]

Intent of the Map

It is important to understand the intended use of the data you are using for mapping and GIS.  Powerful software will allow users to perform many tasks that previously would be very time consuming, but it is imperative that you understand the initial intended use of the data.

For example, the intended use of our Digital Elevation Model is to accurately remove terrain errors in order to produce an accurate digital ortho photograph.  GIS software will allow a GIS technician to produce contours at any interval from this file; the software does not know or care where the dataset is coming from.  It is the responsibility of users of the dataset to know the initial intended uses and accuracy level for the dataset and be prepared for erroneous results if used outside of the data intended purpose.

Another example involves published scale and overlaying data from multiple sources.  In Polk County we have a common basemap which allows our GIS users to overall maps from other local communities without much problem or projection conversions.  You can run into problems and erroneous results when you take data from other sources.  

Common Questions

Q.  Why is the outline of my house shown outside of my property boundary? example

A.  You're viewing the webmap (or hardcopy paper map) from Information Technology with two GIS layers that should not be viewed together.   The external roofline dataset, originally used by insurance companies to calculate storm damage exposure and the Polk County Auditor's parcel polygon layer.  The building roofline dataset’s accuracy is not compatible with the parcel polygon layer and in all cases the spatial location of building rooflines against parcel boundaries should not be relied upon. 

We know displaying incompatible datasets together will only cause confusion to the viewer (most non-GIS people). It was recommended to IT staff that viewing the parcel layer be disabled when the roofline layer is displayed both on-line and in printed copy. Displaying incompatible datasets also has the potential for the public to question the overall accuracy of GIS in Polk County, Iowa.

Rest assured that the Polk County Auditor continues to make every attempt to maintain our cadastral basemap to national map accuracy standards; we would not intentionally display incompatible GIS datasets on our website or on hard copy maps, finally our GIS staff has made several attempts to explain the problem and possible solutions without success to the staff of the Polk County IT Department. The final decision to allow these incompatible GIS datasets to be viewed together rests solely with the Polk County IT Department.

GIS 101 - Know the intended use and accuracy for each layer in your GIS.  Individually, dataset layer accuracy works well within the framework of the original intended use.  Mixing incompatible GIS layers, as in this case, will only lead to confused users and questions about the overall accuracy of the system.   Article  / Second Article  / Third Article / FGDC Standards

Q. Why it is when I measure my lot I come up with a different measurement between two monuments than is listed on my recent plat of survey?

You are most likely measuring your slope distance, your plat lists the horizontal distance.

A. Try measuring again along the horizontal plane, your measurement should be much closer to your survey.

Q.  Where can I get a zip code map? I want to track a spatial relationship by zip code.

A.  Zip codes are used to deliver mail.   Zip codes are linear or points features by nature, they are not polygons.  In general, we don't recommend using zip codes for any GIS projects.  If you must use zip codes for a project, contact the USPS for more information.  link

Q.  Is my property in a flood zone?  

A.  FEMA is the only official source for flood maps. Do not rely on any other source or website.  Visit https://msc.fema.gov/portal for more information, Flood Smart or Call 1-888-379-9531.  

Q.  What is the elevation of a given point?

A.  The USGS has a website that will show the value for elevation from a single Latitude / Longitude point.  

http://viewer.nationalmap.gov/theme/elevation/  

Polk County’s elevation data is used in the ortho-rectification processing for the aerial basemap.  Polk County’s DEM data is not intended to be used to create point or contour elevations.

 


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