1. How do I convert the coordinates of a file?
2. Where can I find map data online?
3. What is PLSS data?
4. What is the description of the BLM PLSS land survey data?
5. What is "protracted" land grid?
6. Is the land survey system in Texas as complex as they say?
7. Why doesn't the map I just imported overlay with my other data?
8. Why doesn't my DEM data line up with the hard-copy topos that I have from the USGS?
9. What is the difference between DEM data and NED data?
10. What is a DOQQ?
11. What is TIGER data?
12. How often are the TIGER files updated?

How do I convert the coordinates of a file?

There are many software packages on the market that will allow you to do this.

Where can I find map data online?

 Please visit our LINKS page for this data.

What is PLSS data?

What is the description of the BLM PLSS land survey data?

Abstract:

This layer is based on Geographic Coordinate Data Base (GCDB) coordinate data.  The locations of Public Land Survey System (PLSS) corners, as represented in geographic coordinate pairs, were derived from a variety of source documents, which include U.S. General Land Office and BLM survey plats/notes, as well as survey data obtained from other U.S. Government agencies, private sector survey firms, and local governments. The attributes assigned to PLSS polygons were taken from the BLM's Legal Land Description (LLD) data set, contained within the Legacy Rehost for the year 2000 (LR2000) automated records system. The coordinate data was produced by using control stations of known location, with varying degrees of accuracy, from various sources which include but are not limited to; U.S. Geological Survey (USGS) topological quadrangles and other sources, National Geodetic Survey (NGS) and US Coast & Geodetic Survey (USC&GS) Cooperative Base Network (CBN) control, Federal Base Network control (FBN), and Continuous Operating Reference Stations (CORS), and Global Positioning System (GPS) data, which is then analyzed and adjusted in concert with official survey data for any given geographic area. The Land Survey Information System Data Base (LSIS) data is a useful representation of the geometry and topology of parcels contained within the PLSS, but its application is intended for mapping purposes only. The GCDB data served from LSIS is not a substitute for a legal land survey.
Purpose:

The GCDB Data  was created to provide the BLM and its public with a set of geographic foundation data that accurately portrays the locations of PLSS corners. The GCDB data is based on the best and most current survey records available and uses known geographic positions of control stations within the PLSS network. This data is a key component of the Land Survey Information System (LSIS) framework upon which parcel boundary information will be assembled.

The Land Survey Information System (LSIS) Data Base was created to provide the BLM and its Public with geographic foundation data from across the country. This includes the Geographic Coordinate Data Base (GCDB). This data most accurately portrays the location of PLSS corners. The LSI is based on the best and most current survey records available and known geographic positions of control stations within the PLSS network. This data will provide the framework upon which the BLM will assemble parcel boundary information for its initial spatially referenced Land Information System (LIS), and will be instrumental in determining where maintenance and upgrade to the PLSS is necessary.

What is "protracted" land grid?

Protracted areas are simply places in the US where no formal surveying has been performed.   Examples include rough terrain (mountains & marshes), land grants, large rivers and drainage systems, indian reservations, and much of Alaska.  In these areas, a land grid must be interpolated without employing traditional surveying techniques.

Is the land survey system in Texas as complex as they say?

Absolutely.   Texas has seen many different governments in its history.   Each government has made its attempt at defining a system for legally partitioning land.   Although each system was different,  they are all maintained and considered in describing land.   Hence we are stuck with blocks, surveys, abstracts, subdivisions, lots, tracts, leagues, labors, metes & bounds, townships, and sections.

Why doesn't the map file I just imported overlay with my other data?

The data that you imported is probably in a different coordinate system than the other data.    Even though you use "UTM meters NAD27,"  for example, if one data set is in UTM zone 14 and the other is in UTM zone 15, these data will not overlay without some coordinate manipulation. The same applies with the State Plane Coordinate System (SPCS). Data in SPCS TX North coordinates will not overlay with data in SPCS TX North Central coordinates. You also need to make sure that you are using common units (ie. meters or feet) as well as common datums (ie. NAD27, NAD83, WGS84, etc.)

Why doesn't my DEM data line up with the hard copy topos that I have from the USGS?

Another good question. Your DEM, NED, and topo elevation data may all be 1:24,000 source data sets. This generally means that the accuracy of all 3 data sets are the same. The complication comes from how your particular software interprets or interpolates the point data of the DEM or NED data sets. Very rarely do 2 different software systems have the same interpolation algorithms (a complex set of computer-based rules on how to interpret a data file) in order to output the same contour lines. As a result, you may have one data set that is off by 5 feet to the north, and another that is off 5 feet to the south. Both data sets are only off by five feet, but there is a 10 feet difference when you compare the two.

What is the difference between DEM data and NED data?

Both data sets express, essentially, the same resolution (30X30 meter spacing between elevation points).   The main difference between the two data sets is that DEM data is based on the USGS 7.5 minute topo quads and the NED data is a seamless coverage of elevation coordinates for the entire U.S.    Generally, each quad of DEM data needs to be fitted to its adjacent quads in order to eliminate the areas of no data between each quad.  Please keep in mind that when you retreive DEM data from the USGS, that it will probable be in the SDTS format.   Please visit our LINKS page for this data.

What is a DOQQ?

DOQQ is an acronym for digital orthophoto quarter quad.   This generally references the NAPP® (national aerial photography program) aerial photos.   These photos are sold by a variety of vendors in many formats and coordinate systems.   Some are black and white photos and some are color infrared.   These files are, sometimes, very large in size, and may very well take up most of your hard disk if you are not careful.   These files may come in a compressed format as well.  For instance, Lizardtech's MRSID® (multi-resolution seamless image database) and ER Mapper's ECW® (enhanced compressed wavelet) formats.

What is TIGER data?

TIGER® is an acronym for the Topologically Integrated Geographic Encoding and Referencing (system or database).  It is a digital geographic database that automates the mapping and related geographic activities required to support the U.S. Census Bureau's census and survey programs.  The content of the TIGER database is undergoing continuous updates and is made available to the public through a variety of TIGER/Line® files that may be obtained free of charge from the internet.  To most of the petroleum industry, this is a mainstream cultural (man made objects) basemap database.    Please visit our LINKS page for this data.

How often are the TIGER files updated?

These files are updated every two years by the US Bureau of Census. Please note that there is about a 12 month lag time before the current data is actually released. For example, TIGER 2000 was released in 2001. Also note that because the Census Bureau collects data from many other government sources, some data are better and more updated than others.